Sometimes, USS files fail to import correctly, requiring manual re-importing. Go to Packages > Luna UI > Runtime > USS.
We have to do this for all folders in here. Make sure to go into each folder and perform this action on the files inside, not on the folder itself. Here's what to do: Select all files by ctrl + a > right click > Reimport.
Do NOT click "Reimport All", make sure you click "Reimport".

Styles should work properly now. If they don't, try reimporting the: Packages > Luna UI > Runtime > Setup > CupkekGamesMainTheme.
Make sure you select the correct Theme.

Assign Luna_InputSystem_Actions as the project wide input settings.

Settings
Theme
Light
Contrast
Material
Dark
Dim
Material Dark
System
Sidebar(Light & Contrast only)
Font Family
DM Sans
Wix
Inclusive Sans
AR One Sans
Direction