fix: Apply dark mode styling to category filter dropdown

[mopik11]

tldr; Ensures the category filter dropdown menu correctly respects dark mode styling by using the appropriate CSS variables. This resolves a visual bug where the dropdown background remained white (light) when the page was viewed in dark mode.

Related issue

• Fixes: Filter by category dropdown unreadable in dark mode (white text on white background) #389

Testing instructions

1. Set your system or browser to Dark Mode (or manually switch to a dark theme on the site).
2. Navigate to the 'All projects' list page.
3. Click the category filter dropdown.
4. Verification: The expanded dropdown menu containing the list of categories (e.g., 'Middleware') should now display with a dark background and light text, consistent with the rest of the page's dark theme.

Browser requirements (for page updates)

Your updates should work in the following environments:

• Latest 2 versions of Edge
• Internet Explorer 11 (should be usable, not pixel perfect)
• Latest 2 versions of Firefox (one on macOS, one of Windows OS)
• Firefox 68 (or latest version for Red Hat Enterprise Linux distribution)
• Latest 2 versions of Chrome (one on macOS, one of Windows OS)
• Latest 2 versions of Safari
• Android mobile device (such as the Galaxy S9)
• Apple mobile device (such as the iPhone X)
• Apple tablet device (such as the iPhone Pro)

Ready-for-merge Checklist

Check off items as they are completed. Feel free to delete items if they are not applicable.

• Expected files: all files in this pull request are related to one request or issue (no stragglers or scope-creep).
• Browser testing passed.
• Repository compiles.
• Documentation (README.md, etc.) updated or added.
• Approved by designer or writer (if applicable).

Merging

fix: Apply dark mode styling to category filter dropdown