Theme designers for both GNOME and KDE make icons more recognisable, colour schemes for every unique set of eyes and ambient light, buttons more readable, menus easier to parse, window controls clearer to discover, and so much more.
Imagine doing all that work and then have it receive nothing but derision.
Theming is accessibility, and with such a renewed focus on accessibility in open source desktops, we really need to have a serious conversation about it again.