Add system light and dark theme support

[ChinmayDaroliya]

Description

Adds a theme selector to the GSoC Guides site with three options:

• System — follows the user's system color scheme (default)
• Light — uses the light theme
• Dark — uses the dark theme

The selected theme is stored in localStorage so it persists across page refreshes.

Changes

• Added a Theme dropdown to the top navigation.
• Added theme switching logic in customscripts.js.
• Added dark-theme styles in css/theme.css.
• Added the theme stylesheet to the site's head include.
• Added support for detecting system theme changes.

Testing

Tested locally with Docker/Jekyll.

Verified:

• System theme
• Light theme
• Dark theme
• Theme persistence after page refresh
• System theme changes
• Navigation/sidebar appearance
• Theme dropdown appearance

Closes #127

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