fix: category dropdown

[huyenltnguyen]

Checklist:

• [x] I have read freeCodeCamp's contribution guidelines.
• [x] My pull request has a descriptive title (not a vague title like Update index.md)

Fixes a couple of issues in the Category dropdown:

• The dropdown color is lighter than the rest of the buttons.
  • 
• Dropdown items text doesn't have sufficient contrast against the background.
  • 

Results

Screenshots

### At rest | Theme | Screenshot | | --- | --- | | Day | | | Dawn | | | Night | | ### Expanded and hovered | Theme | Screenshot | | --- | --- | | Day | | | Dawn | | | Night | |