Linux

It seems like you're thinking about "customization" as "customizable look and feel", but "desktop customization" goes way beyond than that.

The average user isn't going to be going into the guts and changing code. For them customization means the number of exposed "knobs and dials" they can tweak. In that regard KDE is far more customizable than Gnome. I've not played with others enough to evaluate them fairly.

To modify most settings in Gnome, if I remember correctly, you needed an addon. Granted, last time I ran Gnome daily, they were still on Gnome 2 and Ubuntu hadn't come out with Unity yet.

Gnome is technically one of the most customizable, but it is opinionated which means the end user isn’t supposed to change anything, only devs (Like Ubuntu or Zorins customizations). Kde Plasma on the other hand is designed to be easily customizable, you are supposed to make it your own without having to install external software (for example Gnome requires you to manually install the extension manager while it is built in with kde). Then there is Xfce, which is also very customizable but has little built in. This is because with Xfce you are supposed to customize with external software (for example using plank as your dock).

So basically, Gnome is for when you want an opinionated DE (or want to make a distro and have it certain way), KDE is when you want something very customizable and Xfce when you something customizable and modular.

I'd say they all offer different types of customization. It's less a matter of how much you can do, and more a matter of what you want to do and how much time you're willing to spend working on it. KDE is for people who want to customize their desktop, and want it to be easy to do so. GNOME is for people who just want something that works, but it still offers a lot of customization, it's just not as well-supported (their philosophy is "if theming breaks an app, it's not our fault").

KDE doesn't support full CSS customization on its own, but there are theming engines like Kvantum and QtCurve that address the limitations that arise from this. I'd say it's on almost equal footing with GNOME in that regard, since both GTK4+libadwaita and Qt6+KF6 are designed for color scheme customization, but require various workarounds and obscure settings for anything more than that. If anything the workarounds are easier in KDE.

Similarly, KDE supports layout customization through widgets and graphical menus. GNOME also supports layout customization, but through extensions instead.

And then you can do all of the above and more if you use a window manager, or an LXDE/LXQt-style desktop that lets you disable or replace all its components in settings - just mix and match components like panels, file managers, display managers, polkit agents, etc. You can basically build your own DE that way, and it doesn't get much more customizable than that. But maybe you don't want to spend your time choosing every component of your custom DE. That's what something like KDE is for.

CSS is just another tool in a toolbox. Having it doesn't make one more customizable than the other, being more customizable does. Customization means options. Things you can set, things you can change. Not HOW they're changed.

I've gotten a lot out of KDE, honestly. My desktop looks nothing like the default. Panel Colorizer is a great tool.