EDIT: I just noticed in your profile you are using Gimp 3? If so, then scratch everything I said below. Those are for Gimp 2.10.1x (is Gimp 3 really released? Or are you using a nighty version still in development? Gimp 3 will use CSS for stylizing the UI).
Hello Mirek,
this is how Gimp looks for me too with the default Dark theme on Windows. I'm not sure what spacing you expected, but showing us some of those other screenshots you mentioned, where the UI looked fine, would help us see what you mean. Maybe they were using a different theme.
The Gimp UI is controlled by the currently active theme, via a text file called: gtkrc (it is located inside the theme's folder, in your hard-disk). The default themes shipping with Gimp 2.10.1x, are additionally using images for several UI elements, stored in a sub-folder called: ui\). You can change the current theme from within Gimp, via
Edit -> Preferences | Interface | Theme. By default, Gimp now ships with 3 themes: Dark, Gray and Light.
Anyway, I was not happy with the default themes either (at least on Windows, I have no idea how it looks on Linux, but I guess it shouldn't look much different). So I started ported to Gimp 2.10.1x some themes I had made a long time ago for Gimp 2.8. Yesterday I posted a release-candidate version of my Dark (Blue) theme, in this post:
https://www.gimp-forum.net/Thread-Styliz...5#pid18755
Give it a try and see if you like it better than the default theme. Make sure you read the Installation instructions, especially for the Clearlooks theme-engine (my themes need that engine, which doesn't ship with Gimp - without it, the themes will look broken).