Fix folders being light in dark mode.

- Simplifying the styles here, following the same pattern as
  the popup colors.

Bug: 175329686
Test: test wallpaper with light text, light mode
      test again in dark mode
      test wallpaper with dark text, light mode
      test again in dark mode

Change-Id: I04f92bd64de69c5e33fc1bff50c4ecc0e7c17d37