Add Dark Mode Support to NuttX Website

[siddhitripathi25]

Current Behavior:

The NuttX website currently only supports a light theme, which can cause eye strain for users who prefer dark mode, especially during nighttime browsing sessions.

Expected Behavior:

1.Implement a dark mode theme that users can toggle
2.Respect system-level dark mode preferences (prefers-color-scheme)
3.Persist user's theme choice across sessions
4.Ensure all components (code blocks, tables, navigation) are properly styled in both themes

Proposed Solution:

-Add CSS custom properties (variables) for theming
-Implement a theme toggle switch in the header
-Use localStorage to remember user preference
-Support system preference detection
-Create comprehensive dark theme styles for all website elements

Technical Approach:

1.Modify assets/css/main.scss to use CSS variables
2.Add theme toggle in _includes/header.html
3.Implement minimal JavaScript for theme switching
4.Ensure accessibility and proper contrast ratios

Benefits:

1.Improved user experience and accessibility
2.Reduced eye strain for developers
3.Modern web standards compliance
4.Better battery life on OLED devices

[cederom]

I like themed approach, we could have many different color schemes for user to choose, also default like and dark theme depending on the system settings (I use dark themes by default myself). Thanks @siddhitripathi25 :-)

[siddhitripathi25]

@cederom I had raised the pr..Please review it