Add support for dark theme

[preetkaran20]

Currently VulnerableApp only supports light theme mode and doesn't have dark theme mode. This feature is to add darker theme mode or use the current system mode by default.

Ensure all the components are compatible with this change. Please add your thoughts and finalize the design before implementing.

[jayrajsinhdabhi]

Hi @preetkaran20,

I'd like to work on this! Here's my proposed design:
Approach: CSS Custom Properties + System Preference Detection

Three theme modes : light, dark, and system (default). System mode respects prefers-color-scheme from the OS.

CSS Variables : Replace hardcoded colors (#575757, #333, white, #f6f8fa) with custom properties scoped under [data-theme="light"] and [data-theme="dark"] selectors.

RSuite theme switching : RSuite v4 ships both rsuite-default.min.css and rsuite-dark.min.css. I dynamically swap the stylesheet in based on the active theme.

Persistence : Save user's choice in localStorage so it survives page reloads.

Toggle UI : A cycling icon button in the Header navbar (sun → moon → desktop/system).

Component compatibility : All existing CSS files (index.css, Header.css, ChallengeCard.css) updated to use CSS variables.

No new dependencies. Consistent with existing class-based architecture. Let me know if you'd like changes to this approach, otherwise I'll open a PR

[preetkaran20]

sure, please go ahead.