Style Sheets
OkShell uses GTK4, which means you can change the style with css.
Loading a style sheet
Place the css file you want to load in ~/.config/okshell/styles/. Then go to Settings -> Theme -> Custom CSS to select the style sheet you want to load. You can modify your style sheet while using it and see live updates.
Changing global colors and sizing
There are a handful of css variable you can change in :root that will affect the entire shell. You can see the variables here.
Finding CSS classes
Use okshellctl inspect to open the GTK4 inspector.