Unity Style Sheet (USS) files are text files inspired by Cascading Style Sheets (CSS) from HTML. They are primarily used to define the appearance and styles of UI elements.
If you are unfamiliar with the basics, you can refer to Unity's documentation.
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