Feature: add dark mode / theme toggle support to UI

[Harushii16]

What’s missing / problem

The current web interface of the Hacktoberfest Projects Finder has only a light theme. Many users prefer dark mode, especially when browsing at night. There is no built-in dark mode or theme toggle.

Proposed solution

• Introduce CSS (or a CSS-in-JS solution) to support a dark theme (colors, backgrounds, text) for UI components (navbar, cards, background, typography).
• Add a toggle switch (e.g. a button in the header or footer) that lets the user switch between light and dark mode.
• Persist the user’s choice (e.g. via localStorage) so when they revisit, their preferred theme remains.
• Ensure that components (cards, buttons, text) are styled in both themes correctly (contrast, readability).
• Optionally: use CSS custom properties (CSS variables) for colors so it's easier to switch.

Acceptance criteria

• There is a toggle visible in the UI (header/footer) to switch theme.
• The dark theme is applied instantly when toggled.
• The selected theme persists across page reload / navigations.
• All UI elements remain readable and consistent in dark mode (no inverted images that break, text contrast okay).
• The default is light theme (unchanged) for users who don’t toggle.

I can prepare a PR with the required CSS changes, toggle logic, and small persistence code.

[max-programming]

This website follows hacktoberfest.com and I think they only have dark mode (no only light mode)
Following theme of that site right now is the best thing we can do

If you want to enhance anything specifically in the design, do open a pr