Change font weight of menu items to 400 instead of 200, improves readability

Fixes #9828.

Things to consider:
   - May not be the only place in the UI that this error happens.
   - Should the font weight used (400) be put into a LESS variable? If so, how should that variable be named?
      - We already have `@font-weight-semibold: 500` and `@font-weight-light: 200`.