mkdir -p ~/.warp/themes/
Note: It may take several minutes for Warp to initially discover the new config directory. You can either wait or just restart the application. After that step, all future changes to
~/.warp/themesdirectory will be reflected in Warp within seconds.
cp ~/Downloads/sarcastic-white.yaml ~/.warp/themes/
Your new theme should now be visible on the list(Settings → Appearance) of available themes.
Refer to the Warp documentation for more info on custom themes.