diff --git a/src/components/ui/badge.tsx b/src/components/ui/badge.tsx
index 65e9e7d..ed2a948 100644
--- a/src/components/ui/badge.tsx
+++ b/src/components/ui/badge.tsx
@@ -3,17 +3,22 @@ import { cva, type VariantProps } from "class-variance-authority";
 import { cn } from "@/lib/utils";
 
 const badgeVariants = cva(
-  "inline-flex items-center rounded-full border px-2.5 py-0.5 text-xs font-semibold transition-colors",
+  "inline-flex items-center border px-2.5 py-0.5 text-xs font-semibold",
   {
     variants: {
       variant: {
-        default: "border-slate-200 bg-slate-100 text-slate-700",
-        accent: "border-emerald-200 bg-emerald-100 text-emerald-800",
-        danger: "border-red-200 bg-red-100 text-red-700"
+        default: "border-transparent bg-muted text-muted-foreground",
+        accent: "border-transparent bg-accent text-accent-foreground",
+        danger: "border-transparent bg-destructive/10 text-destructive"
+      },
+      shape: {
+        pill: "rounded-full",
+        square: "rounded-md"
       }
     },
     defaultVariants: {
-      variant: "default"
+      variant: "default",
+      shape: "pill"
     }
   }
 );
@@ -21,8 +26,8 @@ const badgeVariants = cva(
 export interface BadgeProps
   extends React.HTMLAttributes<HTMLDivElement>, VariantProps<typeof badgeVariants> {}
 
-function Badge({ className, variant, ...props }: BadgeProps): JSX.Element {
-  return <div className={cn(badgeVariants({ variant }), className)} {...props} />;
+function Badge({ className, variant, shape, ...props }: BadgeProps): JSX.Element {
+  return <div className={cn(badgeVariants({ variant, shape }), className)} {...props} />;
 }
 
 export { Badge, badgeVariants };
diff --git a/src/components/ui/button.tsx b/src/components/ui/button.tsx
index 7270d2e..eed42fb 100644
--- a/src/components/ui/button.tsx
+++ b/src/components/ui/button.tsx
@@ -3,19 +3,22 @@ import { cva, type VariantProps } from "class-variance-authority";
 import { cn } from "@/lib/utils";
 
 const buttonVariants = cva(
-  "inline-flex items-center justify-center rounded-md text-sm font-semibold transition-colors focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-emerald-600/50 disabled:pointer-events-none disabled:opacity-50",
+  "inline-flex items-center justify-center gap-2 rounded-md text-sm font-semibold transition-[transform,background-color,box-shadow] duration-200 ease-swift focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-ring focus-visible:ring-offset-2 focus-visible:ring-offset-background active:scale-[0.98] disabled:pointer-events-none disabled:cursor-not-allowed disabled:opacity-50 [&_svg]:size-4 [&_svg]:shrink-0",
   {
     variants: {
       variant: {
-        default: "bg-emerald-700 text-emerald-50 hover:bg-emerald-800",
-        secondary: "bg-slate-100 text-slate-900 hover:bg-slate-200 border border-slate-200",
-        ghost: "text-slate-700 hover:bg-slate-100",
-        destructive: "bg-red-600 text-white hover:bg-red-700"
+        default:
+          "bg-primary text-primary-foreground shadow-[inset_0_1px_0_0_hsl(0_0%_100%/0.12)] hover:bg-primary-hover",
+        secondary:
+          "border border-border bg-secondary text-secondary-foreground hover:bg-secondary-hover",
+        ghost: "text-secondary-foreground hover:bg-secondary",
+        destructive:
+          "bg-destructive text-destructive-foreground shadow-[inset_0_1px_0_0_hsl(0_0%_100%/0.15)] hover:bg-destructive-hover"
       },
       size: {
         default: "h-10 px-4 py-2",
-        sm: "h-9 px-3",
-        lg: "h-11 px-6"
+        sm: "h-9 px-3 text-[13px]",
+        lg: "h-11 px-6 text-base"
       }
     },
     defaultVariants: {
diff --git a/src/components/ui/card.tsx b/src/components/ui/card.tsx
index 0a588e3..cfb3932 100644
--- a/src/components/ui/card.tsx
+++ b/src/components/ui/card.tsx
@@ -5,7 +5,7 @@ function Card({ className, ...props }: React.HTMLAttributes<HTMLDivElement>): JS
   return (
     <div
       className={cn(
-        "rounded-xl border border-slate-200 bg-white text-slate-950 shadow-[0_8px_24px_rgba(15,23,42,0.06)]",
+        "rounded-lg border border-border bg-card text-card-foreground shadow-[0_1px_0_0_hsl(0_0%_100%/0.6)_inset,0_8px_24px_hsl(222_47%_11%/0.06)]",
         className
       )}
       {...props}
@@ -17,15 +17,19 @@ function CardHeader({ className, ...props }: React.HTMLAttributes<HTMLDivElement
   return <div className={cn("flex flex-col space-y-1.5 p-4", className)} {...props} />;
 }
 
-function CardTitle({ className, ...props }: React.HTMLAttributes<HTMLHeadingElement>): JSX.Element {
-  return <h3 className={cn("text-lg font-extrabold tracking-tight", className)} {...props} />;
+type CardTitleProps = React.HTMLAttributes<HTMLHeadingElement> & {
+  as?: "h1" | "h2" | "h3" | "h4";
+};
+
+function CardTitle({ className, as: Tag = "h3", ...props }: CardTitleProps): JSX.Element {
+  return <Tag className={cn("text-lg font-bold tracking-tight", className)} {...props} />;
 }
 
 function CardDescription({
   className,
   ...props
 }: React.HTMLAttributes<HTMLParagraphElement>): JSX.Element {
-  return <p className={cn("text-sm text-slate-600", className)} {...props} />;
+  return <p className={cn("text-sm text-muted-foreground", className)} {...props} />;
 }
 
 function CardContent({ className, ...props }: React.HTMLAttributes<HTMLDivElement>): JSX.Element {
diff --git a/src/components/ui/input.tsx b/src/components/ui/input.tsx
index 476af94..a0cc470 100644
--- a/src/components/ui/input.tsx
+++ b/src/components/ui/input.tsx
@@ -9,7 +9,7 @@ const Input = React.forwardRef<HTMLInputElement, InputProps>(
       <input
         type={type}
         className={cn(
-          "flex h-10 w-full rounded-md border border-slate-300 bg-white px-3 py-2 text-sm text-slate-900 shadow-sm transition-colors file:border-0 file:bg-transparent file:text-sm file:font-medium placeholder:text-slate-400 focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-emerald-600/50 disabled:cursor-not-allowed disabled:opacity-50",
+          "flex h-10 w-full rounded-md border border-input bg-card px-3 py-2 text-sm text-foreground shadow-sm transition-colors file:border-0 file:bg-transparent file:text-sm file:font-medium placeholder:text-muted-foreground focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-ring focus-visible:ring-offset-2 focus-visible:ring-offset-background disabled:cursor-not-allowed disabled:opacity-50 aria-[invalid=true]:border-destructive aria-[invalid=true]:focus-visible:ring-destructive",
           className
         )}
         ref={ref}
diff --git a/src/components/ui/select.tsx b/src/components/ui/select.tsx
index 8b40ae6..8b9aa2c 100644
--- a/src/components/ui/select.tsx
+++ b/src/components/ui/select.tsx
@@ -5,14 +5,28 @@ export type SelectProps = React.SelectHTMLAttributes<HTMLSelectElement>;
 
 const Select = React.forwardRef<HTMLSelectElement, SelectProps>(({ className, ...props }, ref) => {
   return (
-    <select
-      className={cn(
-        "flex h-10 w-full rounded-md border border-slate-300 bg-white px-3 py-2 text-sm text-slate-900 shadow-sm focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-emerald-600/50 disabled:cursor-not-allowed disabled:opacity-50",
-        className
-      )}
-      ref={ref}
-      {...props}
-    />
+    <div className="relative">
+      <select
+        className={cn(
+          "flex h-10 w-full appearance-none rounded-md border border-input bg-card px-3 py-2 pr-9 text-sm text-foreground shadow-sm transition-colors focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-ring focus-visible:ring-offset-2 focus-visible:ring-offset-background disabled:cursor-not-allowed disabled:opacity-50 aria-[invalid=true]:border-destructive aria-[invalid=true]:focus-visible:ring-destructive",
+          className
+        )}
+        ref={ref}
+        {...props}
+      />
+      <svg
+        aria-hidden="true"
+        viewBox="0 0 24 24"
+        fill="none"
+        stroke="currentColor"
+        strokeWidth="2"
+        strokeLinecap="round"
+        strokeLinejoin="round"
+        className="pointer-events-none absolute right-3 top-1/2 size-4 -translate-y-1/2 text-muted-foreground"
+      >
+        <path d="m6 9 6 6 6-6" />
+      </svg>
+    </div>
   );
 });
 Select.displayName = "Select";
diff --git a/src/components/ui/separator.tsx b/src/components/ui/separator.tsx
index 4b7191f..1659718 100644
--- a/src/components/ui/separator.tsx
+++ b/src/components/ui/separator.tsx
@@ -3,18 +3,24 @@ import { cn } from "@/lib/utils";
 
 interface SeparatorProps extends React.HTMLAttributes<HTMLDivElement> {
   orientation?: "horizontal" | "vertical";
+  decorative?: boolean;
 }
 
 function Separator({
   className,
   orientation = "horizontal",
+  decorative = true,
   ...props
 }: SeparatorProps): JSX.Element {
   return (
     <div
+      role={decorative ? "none" : "separator"}
+      aria-orientation={decorative ? undefined : orientation}
       className={cn(
-        "shrink-0 bg-slate-200",
-        orientation === "horizontal" ? "h-px w-full" : "h-full w-px",
+        "shrink-0",
+        orientation === "horizontal"
+          ? "h-px w-full bg-gradient-to-r from-transparent via-border to-transparent"
+          : "h-full w-px bg-gradient-to-b from-transparent via-border to-transparent",
         className
       )}
       {...props}
diff --git a/src/components/ui/textarea.tsx b/src/components/ui/textarea.tsx
index 548e6e9..91db60f 100644
--- a/src/components/ui/textarea.tsx
+++ b/src/components/ui/textarea.tsx
@@ -8,7 +8,7 @@ const Textarea = React.forwardRef<HTMLTextAreaElement, TextareaProps>(
     return (
       <textarea
         className={cn(
-          "flex min-h-[80px] w-full rounded-md border border-slate-300 bg-white px-3 py-2 text-sm text-slate-900 shadow-sm placeholder:text-slate-400 focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-emerald-600/50 disabled:cursor-not-allowed disabled:opacity-50",
+          "flex min-h-[80px] w-full rounded-md border border-input bg-card px-3 py-2 text-sm text-foreground shadow-sm transition-colors placeholder:text-muted-foreground focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-ring focus-visible:ring-offset-2 focus-visible:ring-offset-background disabled:cursor-not-allowed disabled:opacity-50 aria-[invalid=true]:border-destructive aria-[invalid=true]:focus-visible:ring-destructive",
           className
         )}
         ref={ref}
diff --git a/src/editor/main.tsx b/src/editor/main.tsx
index 716ff06..d80733f 100644
--- a/src/editor/main.tsx
+++ b/src/editor/main.tsx
@@ -579,7 +579,7 @@ function EditorApp(): JSX.Element {
       <Card className="lg:max-h-[calc(100vh-2.5rem)] lg:overflow-auto">
         <CardHeader className="space-y-3">
           <div className="flex items-center justify-between">
-            <CardTitle>Shotback Editor</CardTitle>
+            <CardTitle as="h1">Shotback Editor</CardTitle>
             <Badge variant="accent">{annotations.length} notes</Badge>
           </div>
           <Button disabled={isBusy} onClick={() => void takeScreenshot()}>
@@ -618,7 +618,13 @@ function EditorApp(): JSX.Element {
             <span className="text-xs font-semibold uppercase tracking-wide text-slate-500">
               Color
             </span>
-            <Input type="color" value={color} onChange={(event) => setColor(event.target.value)} />
+            <Input
+              type="color"
+              aria-label="Annotation color"
+              value={color}
+              onChange={(event) => setColor(event.target.value)}
+              className="h-10 cursor-pointer p-1"
+            />
           </label>
 
           <label className="block space-y-1.5">
@@ -688,7 +694,7 @@ function EditorApp(): JSX.Element {
           <Separator />
           <section className="space-y-2">
             <div className="flex items-center justify-between">
-              <h4 className="m-0 text-sm font-semibold">Comment Timeline</h4>
+              <h2 className="m-0 text-sm font-semibold">Comment Timeline</h2>
               <Badge>{timelineItems.length}</Badge>
             </div>
             {timelineItems.length === 0 ? (
@@ -754,7 +760,7 @@ function EditorApp(): JSX.Element {
           <Separator />
           <section className="space-y-2">
             <div className="flex items-center justify-between">
-              <h4 className="m-0 text-sm font-semibold">Saved Shares</h4>
+              <h2 className="m-0 text-sm font-semibold">Saved Shares</h2>
               <div className="flex items-center gap-2">
                 <Badge>{savedShares.length}</Badge>
                 {savedShares.length > 0 ? (
diff --git a/src/popup/main.tsx b/src/popup/main.tsx
index d39d955..2284846 100644
--- a/src/popup/main.tsx
+++ b/src/popup/main.tsx
@@ -26,8 +26,10 @@ function PopupApp(): JSX.Element {
       <Card>
         <CardHeader className="space-y-2 pb-3">
           <div className="flex items-center justify-between gap-2">
-            <CardTitle className="text-3xl leading-none">Shotback</CardTitle>
-            <Badge variant="accent" className="shrink-0">
+            <CardTitle as="h1" className="text-3xl leading-none">
+              Shotback
+            </CardTitle>
+            <Badge variant="accent" shape="square" className="shrink-0">
               New
             </Badge>
           </div>
diff --git a/src/styles/globals.css b/src/styles/globals.css
index 99c493c..a865aa0 100644
--- a/src/styles/globals.css
+++ b/src/styles/globals.css
@@ -8,11 +8,33 @@
   :root {
     --background: 210 40% 98%;
     --foreground: 222 47% 11%;
+
     --card: 0 0% 100%;
     --card-foreground: 222 47% 11%;
-    --border: 214 32% 91%;
-    --ring: 160 84% 39%;
-    --radius: 0.8rem;
+
+    --primary: 163 94% 24%; /* emerald-700 */
+    --primary-hover: 162 88% 20%; /* emerald-800 */
+    --primary-foreground: 152 81% 96%; /* emerald-50 */
+
+    --secondary: 210 40% 96%; /* slate-100 */
+    --secondary-hover: 214 32% 91%; /* slate-200 */
+    --secondary-foreground: 222 47% 11%; /* slate-900 */
+
+    --muted: 210 40% 96%; /* slate-100 */
+    --muted-foreground: 215 16% 47%; /* slate-500 */
+
+    --accent: 149 80% 90%; /* emerald-100 */
+    --accent-foreground: 162 88% 20%; /* emerald-800 */
+
+    --destructive: 0 72% 51%; /* red-600 */
+    --destructive-hover: 0 74% 42%; /* red-700 */
+    --destructive-foreground: 0 0% 100%;
+
+    --border: 214 32% 91%; /* slate-200 — structural hairlines */
+    --input: 213 27% 84%; /* slate-300 — form-control borders */
+    --ring: 161 94% 30%; /* emerald-600 — focus ring */
+
+    --radius: 0.75rem;
   }
 
   * {
@@ -28,3 +50,38 @@
       hsl(var(--background));
   }
 }
+
+/*
+ * Dark theme tokens. Kept OUTSIDE @layer base so Tailwind does not tree-shake
+ * the `.dark` selector (no `dark` reference exists in content yet). Opt-in:
+ * add `class="dark"` to <html>. Light remains the default.
+ */
+.dark {
+  --background: 222 47% 6%;
+  --foreground: 210 40% 96%;
+
+  --card: 222 44% 9%;
+  --card-foreground: 210 40% 96%;
+
+  --primary: 161 94% 30%; /* emerald-600 */
+  --primary-hover: 161 94% 36%;
+  --primary-foreground: 222 47% 6%;
+
+  --secondary: 215 25% 17%; /* slate-800 */
+  --secondary-hover: 215 19% 22%;
+  --secondary-foreground: 210 40% 96%;
+
+  --muted: 215 25% 17%;
+  --muted-foreground: 215 20% 65%; /* slate-400 */
+
+  --accent: 162 88% 16%;
+  --accent-foreground: 152 76% 80%; /* emerald-200 */
+
+  --destructive: 0 72% 51%;
+  --destructive-hover: 0 74% 58%;
+  --destructive-foreground: 0 0% 100%;
+
+  --border: 215 25% 17%;
+  --input: 215 25% 20%;
+  --ring: 161 94% 36%;
+}
diff --git a/src/viewer/main.tsx b/src/viewer/main.tsx
index 8ff624c..a0ca522 100644
--- a/src/viewer/main.tsx
+++ b/src/viewer/main.tsx
@@ -45,7 +45,7 @@ function ViewerApp(): JSX.Element {
       <main className="mx-auto min-h-screen w-full max-w-5xl p-4 md:p-6">
         <Card>
           <CardHeader>
-            <CardTitle>Shotback Share</CardTitle>
+            <CardTitle as="h1">Shotback Share</CardTitle>
             <CardDescription>{status}</CardDescription>
           </CardHeader>
         </Card>
@@ -63,7 +63,7 @@ function ViewerApp(): JSX.Element {
         <Card>
           <CardHeader className="space-y-3">
             <div className="flex flex-wrap items-center justify-between gap-2">
-              <CardTitle>Shotback Share</CardTitle>
+              <CardTitle as="h1">Shotback Share</CardTitle>
               <Badge variant="accent">Local</Badge>
             </div>
             <CardDescription>
@@ -93,7 +93,7 @@ function ViewerApp(): JSX.Element {
         <Card>
           <CardHeader className="pb-3">
             <div className="flex items-center justify-between gap-3">
-              <CardTitle className="text-base">Annotated Image</CardTitle>
+              <CardTitle as="h2" className="text-base">Annotated Image</CardTitle>
               <Button
                 variant="secondary"
                 size="sm"
diff --git a/tailwind.config.js b/tailwind.config.js
index 49b8236..833cf7f 100644
--- a/tailwind.config.js
+++ b/tailwind.config.js
@@ -1,20 +1,50 @@
 /** @type {import('tailwindcss').Config} */
 export default {
+  darkMode: "class",
   content: ["./popup.html", "./editor.html", "./viewer.html", "./src/**/*.{ts,tsx}"],
   theme: {
     extend: {
       colors: {
         background: "hsl(var(--background))",
         foreground: "hsl(var(--foreground))",
-        card: "hsl(var(--card))",
-        "card-foreground": "hsl(var(--card-foreground))",
+        card: {
+          DEFAULT: "hsl(var(--card))",
+          foreground: "hsl(var(--card-foreground))"
+        },
+        primary: {
+          DEFAULT: "hsl(var(--primary))",
+          hover: "hsl(var(--primary-hover))",
+          foreground: "hsl(var(--primary-foreground))"
+        },
+        secondary: {
+          DEFAULT: "hsl(var(--secondary))",
+          hover: "hsl(var(--secondary-hover))",
+          foreground: "hsl(var(--secondary-foreground))"
+        },
+        muted: {
+          DEFAULT: "hsl(var(--muted))",
+          foreground: "hsl(var(--muted-foreground))"
+        },
+        accent: {
+          DEFAULT: "hsl(var(--accent))",
+          foreground: "hsl(var(--accent-foreground))"
+        },
+        destructive: {
+          DEFAULT: "hsl(var(--destructive))",
+          hover: "hsl(var(--destructive-hover))",
+          foreground: "hsl(var(--destructive-foreground))"
+        },
         border: "hsl(var(--border))",
+        input: "hsl(var(--input))",
         ring: "hsl(var(--ring))"
       },
       borderRadius: {
         lg: "var(--radius)",
         md: "calc(var(--radius) - 2px)",
         sm: "calc(var(--radius) - 4px)"
+      },
+      transitionTimingFunction: {
+        swift: "cubic-bezier(0.32, 0.72, 0, 1)"
       }
     }
   },