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?
As far as KDE vs. GNOME is concerned: KDE contains a lot of customizable features as an expectation and thus has great support for a wide array of customization. Both KDE and GNOME are extensible, with third-party extensions to extend or change functionality available. What makes GNOME less customizable, albeit supporting stylesheets and extensions, both are not expected to be used in any form (outside of defaults provided via Adwaita), and neither do many independent apps written in GTK3, GTK4. GNOME offers fairly minimal customization options without resorting to GNOME Tweaks, third-party extensions, and unsupported customized themes: all things that can break GNOME as while the customization does exist, the developers don’t embrace it and have no expectation to not break it with any update.