Linux

You very likely want KDE. It's got the most features out of the box, each of which is something you can customize (whether you want it enabled and with what behaviour).

There's arguments to be made that more minimal desktops, like Xfce, or even a DIY setup using a window manager, are simpler to customize, because they have fewer features.
For example, the panel theme in KDE involves various colors and icons and whatnot, so you need a full-fledged theme package to customize it, even if you 'just' want to change the panel color. In Xfce, the panel color is a simple hex value that you can just set.

GNOME is a weird one. It also has relatively many features, so it does still have relatively many options. Some configurations are relegated behind extensions or a separate settings program (GNOME Tweak Tool). And yes, you can technically apply CSS. But then for some relatively basic things, you need to dip into CSS or other workarounds.
I don't know, if it's still the case, but a few years ago, I just wanted my windows to not have a titlebar, which took me three days to figure out how to do, eventually solving it with CSS.
A few months later, I switched to KDE and there it can be done with a GUI setting, which took me less than ten minutes to figure out.