feat(ui): dark mode

[almajd3713]

Summary of Changes

Light theme burned my eyes, so added a theme selector and dark mode

Terminals and things already in dark mode were not affected.

Types of Changes

• New feature / node type addition
• Bug fix (non-breaking change resolving an issue)
• Refactoring / structural cleanup
• Documentation update

Verification & Testing

Automated Checks

• Run npm run lint successfully with no errors
• Run npm run build successfully with no compilation errors
• Run npm test successfully (all tests pass)

Manual Verification

Tested in UI, site works as usual with no issues. Variables can be changed in css if the proposed theme wasn't suitable.

Checklist

• My code follows the repository's code style and lint standards
• I have updated the documentation or instructions if necessary
• All unit and integration tests are passing

[almajd3713]

I did go through everything to make sure all is well, but it'd be great to get any feedback in case I could've implemented things differently in a better manner.