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OK, then I do not know why. I find that KDE is heavy handed to non-KDE Qt apps. And often when I have a perfectly nice looking COSMIC or GNOME desktop. if I install and log into KDE; then, when I switch back to COSMIC/GNOME it looks wrong. Desktop Linux fragmentation is hostile to app developers who are already spread thin and have better things to work on. You should simply use the Fusion Dark theme. I will let someone else submit a pull request if they want to fix it.
alr thx
Regarding the original post and screenshot, this is due to KDE Settings > Colors & Themes > Application Style > Configure Icons and Toolbars > Main toolbar label: Beside icons. You can change it to None. I see that in our code, we configure these toolbars with ToolButtonFollowStyle, but for the next version I have change that to what our UI layouts demand: ToolButtonIconOnly

