fix: theme toggle button in nav isn't intuitive

[davis9001]

As a user I want the light/dark theme toggle button in the navigation bar to work as follows:

• If there is no preference already set then default to browser/system setting
• Clicking the toggle button should switch to the opposite of the current theme (light/dark)
• The theme toggle button should show a sun icon when in dark mode and a moon icon when in light mode (indicating what the button will switch to)
• The theme toggle button should show the theme colors for the opposing light/dark theme, so if we're in light mode then the theme toggle button should use dark mode colors
• If the user clicks/presses and holds on the toggle button a modal should appear offering all of the theme options (light/dark/system/etc.)