Implement responsive dark mode theme controller with localStorage persistence (#437)

[Aryan0819]

Which issue does this PR close?

• Closes Adding a Dark Mode Toggle to Student Dashboard #437

Rationale for this change

The student achievements dashboard currently only supports a bright, light-themed user interface. This can cause eye strain during extended nighttime viewing sessions. Additionally, the existing "#mode-toggle" button is present in the HTML markup but lacks the backend or frontend logic to actively modify system theme parameters. Implementing a client-side theme controller solves this by offering a persistent theme preference without breaking existing layout code.

What changes are included in this PR?

• styles.css: Added base CSS variable mapping configurations under :root for the default dark theme parameters and explicit class variable overrides under body.light-mode. Introduced a global smooth transition rule for background-color, color, and border-color fields to prevent harsh snapping flashes when themes toggle.
• script.js: Implemented a client-side theme manager utilizing an IIFE execution flow to eliminate layout screen-flash on DOM startup. Added accessibility support via ARIA attributes and active cross-tab theme state syncing using the Window storage event listener.

Are these changes tested?

Yes, manually verified locally across multiple browser environments:

1. Persistence Test: Verified that selecting a theme preference accurately binds to the HTML5 localStorage state repository and holds its exact parameters across manual hard page-reloads.
2. System Preference Test: Verified that clearing local cache data successfully triggers fallback listeners to seamlessly match the operating system level color rules (prefers-color-scheme).
3. Cross-Tab Synchronization Test: Confirmed that opening up multiple dashboard tabs simultaneously and changing the theme state in one instantly updates all alternative tab environments via window storage observers.

Are there any user-facing changes?

Yes. Users will now see a fully interactive theme control button in the top corner layout. Clicking this button dynamically adjusts the entire interface theme spectrum fluidly between light and dark visual aesthetics.

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[github-actions]

Thanks for creating a PR for your Issue! ☺️

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In the meantime, please double-check the file changes and ensure that all commits are accurate.

If there are any unresolved review comments, feel free to resolve them. 🙌🏼

[entelligence-ai-pr-reviews]

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Confidence Score: 2/5 - Changes Needed

Not safe to merge — this PR introduces duplicate :root and body.light-mode CSS blocks in static/styles.css that silently overwrite the existing dark-theme and light-theme variable definitions with conflicting values, which will cause unpredictable theming behavior at runtime. The PR's goal of implementing a responsive dark mode controller with localStorage persistence is a worthwhile feature, but the CSS conflicts mean the theme variables that the controller reads and applies will be unreliable depending on cascade order. Both duplicate blocks must be merged into their respective existing declarations before this is safe to ship.

Key Findings:

• The duplicate :root block in static/styles.css redefines the same four CSS custom properties (likely --bg-color, --text-color, etc.) already declared at line 3, with different values — whichever block appears last in the file wins, meaning the original intended dark-theme defaults are silently discarded.
• The duplicate body.light-mode block overwrites the same variables already defined at lines 14–23 with conflicting values, so the light-mode theme applied by the JavaScript controller will not match the intended design — this is a functional bug, not a cosmetic one.
• Because the theme controller uses localStorage persistence, users who have already saved a theme preference will see the wrong colors on page load until the CSS is corrected and they clear their stored value, making this a potential regression for returning users.

Files requiring special attention

• static/styles.css

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[entelligence-ai-pr-reviews]

EntelligenceAI PR Summary

This PR enforces strict cross-origin image handling in the AchievementExporter class by explicitly disabling the allowTaint option in both html2canvas calls.

• Set allowTaint: false in both html2canvas invocations in static/js/achievement-export.js
• Aligns allowTaint behavior with the existing useCORS: true configuration
• Prevents cross-origin images from tainting the canvas context

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Confidence Score: 5/5 - Safe to Merge

Safe to merge — this PR makes a targeted, low-risk hardening change to achievement-export.js by explicitly setting allowTaint: false in both html2canvas invocations, aligning the configuration with the existing useCORS: true option. The change is purely defensive: allowTaint defaults to false in html2canvas, so this is a clarifying explicit declaration rather than a behavioral change, meaning no regression risk is introduced. No review comments were generated and full file coverage was achieved, giving high confidence in the correctness of this minimal diff.

Key Findings:

• Setting allowTaint: false alongside useCORS: true is the correct and recommended html2canvas configuration for handling cross-origin images securely — these two options are complementary and the PR correctly applies them consistently across both canvas render calls.
• Because allowTaint defaults to false in html2canvas, this change is idempotent in behavior but improves code clarity and guards against any future library version that might change the default — a net positive with zero downside risk.
• The PR title/summary mismatch (title references 'dark mode theme controller' while the actual change is about CORS/canvas) is a documentation oddity worth noting, but it does not affect the code correctness or safety of the diff itself.

Files requiring special attention

• static/js/achievement-export.js

[Aryan0819]

Please review this

[Aryan0819]

@pratyushjha06 @mahi-8758 please review it

[mahi-8758]

@Aryan0819 okk

[mahi-8758]

This PR is reviewed and has no conflicts.

[Aryan0819]

@mahi-8758 can you please merge this pr and add required lables so that i got my gssoc points

[Aryan0819]

@anika-p @devmdave please merge this pr and add the required lables