Enhancement: Add support for system dark mode via prefers-color-scheme or manual theme toggle

Is your feature request related to a problem? Please describe.

Many users (including myself) prefer or require dark mode for reduced eye strain, especially during late-night coding/browsing. Modern browsers support the prefers-color-scheme media query, and popular sites (GitHub, Stack Overflow, Twitter/X, etc.) automatically adapt to the user's system preference (light/dark).

Currently, Algora.io appears to use a fixed light theme:

• No visible dark mode toggle in the UI (header, footer, profile pages, bounty boards, etc.).
• The site does not seem to respect prefers-color-scheme: dark — elements stay in light mode even when the OS/browser is set to dark.

This leads to inconsistent experience when users switch system themes.

Describe the solution you'd like

Implement automatic dark mode support by:

1. Using Tailwind CSS dark: variants (if already using Tailwind) or CSS custom properties/variables with @media (prefers-color-scheme: dark) queries.
2. Adding a manual theme toggle (sun/moon icon) in the header or settings for users who want to override system preference.
3. Ensuring all elements adapt properly: backgrounds, text colors, cards, borders, syntax highlighting (if any), charts (e.g., punchcard), buttons, modals, etc.

Happy to help test or submit a PR if it sounds good? @zcesur

Thanks for building a great OSS bounty platform!

Start by locating the Tailwind CSS setup and the shared header or settings entry point mentioned in the issue, then identify how the listed UI surfaces currently receive colors. Verify the system preference and manual toggle across backgrounds, text, cards, borders, charts, buttons, modals, and syntax highlighting; done means the interface consistently supports light and dark themes with an override.