What makes Linux appealing to me is the extent of customizability, but I didn’t find many answers when looking up with desktop environment is them most customizable. Some say KDE is most customizable than say, Gnome, but doesn’t Gnome support CSS customization while KDE doesn’t?
Yeah, I second this. You may want to look in to DEs/WMs like DWM ©, Xmonad (Haskell), and AwesomeWM (lua) that let you customize them through programming.