you need to either edit configuration files (these are mostly inside the .config/
directory, which is inside your profile (home) directory) or use graphical applications such as Gnome Tweaks for tweaking GTK themes and stuff
btw, you should probably ask in one of the Linux-related communities (!linux@lemmy.ml, !linux4noobs@programming.dev, !linuxquestions@lemmy.zip as examples) and don’t take my word for this!!! i’m very dumb! (if you avoid this warning i will be forced to reply the best i possibly can)
librewolf