There is no single desktop environment on linux. Your customization begins with choosing your desktop environment/window manager.
To get a feeling for the things people like to do, you could take a look at !unixporn@lemmy.ml or !unixporn@lemmy.world
This is a choice as well. Some people like editing configuration files, while others prefer a gui. The big desktop environments like gnome or kde of course offer changing themes and wallpapers in a gui.
You can look at some themes for kde plasma on this site: https://store.kde.org/browse?cat=104