The short answer is because any flag will break other things. There is another suggestion to not require any flag, but it probably will not be implemented becau

The short answer is because any flag will break other things. There is another suggestion to not require any flag, but it probably will not be implemented because of this: https://gitlab.gnome.org/GNOME/gtk/-/issues/5026 https://gitlab.gnome.org/GNOME/gtk/-/issues/5026 It is not possible in open source to force everyone to use one toolkit, in my opinion you are wasting your time by wishing for that. There are dozens of toolkits to choose from and the number seems to be growing. The fragmentation has only gotten stronger over the last 25 years, if Qt was the clear winner the proliferation would have stopped long ago. Now there is pretty much no chance that you will be able to hack every toolkit to get all of them to look and work consistently. In particular, the way an app works is decided by the HIG it follows, not the theme. This is reflected in the entire design of the app, to change it requires redesigning the whole app. It cannot be changed just by setting some environment variables.