Theming GTK2 and GTK3 applications was simple. Applying custom themes to GTK4/libadwaita apps is unsupported. You can enable dark mode and change the accent colours, but that's about it.

KDE/Qt supports theming out of the box, but theming apps outside KDE is painful. You have to juggle with several utilities (qt5ct, qt6ct, Kvantum manager) and many environmental variables. For example, Krita doesn't respect the theme you set using qt5ct unless you set KRITA_NO_STYLE_OVERRIDE=1. Using qt6ct I can make Okular (and other Qt6 apps) use the style that emulates Windows 95, but I can't make them use a custom colour palette, so they stick out.

The worst thing you could do is try to make GTK and Qt apps look consistent.

Waiting for the day the question becomes:

Qt or Cosmic?

☺️

Both can be customized, and a combination of both is most customizeable
But I prefer QT

Qt6 is a lot more customisable than GTK 4, but you probably won't notice if:

• You're using GTK 2/3
• You're using a boring flat theme

However, if you're using Flatpaks, GTK is possibly better.

For GTK there is this cool project, ASTAL:

https://aylur.github.io/astal/

TUI is the only truly satisfactory way.