There is one thing I would like somebody here to at least confirm. When I open "Control Center", "Appearance", theme "Custom" (the first theme in the list) is selected, even though I never customised anything in this respect.
If I then click on another theme, like "Yaru-bark", then the "Custom" theme is selected again. However, a second click on "Yaru-bark" does make it selected.
Say I close the "Appearance" window, and open it again. Then the "Custom" theme is selected again!
If I then install package mate-themes, I get other themes like "Menta". I believe that those are GTK 2 themes, but I am no expert in this area. In any case, they behave differently: when I select one of those themes, then the "Custom" theme disappears, and there is no strange behaviour anymore.
custom assumes you will make changes to the current theme and save that. If you had made no changes in Custom then the current theme displayed is the same as you picked previously and custom is the same theme as well, providing you don't change the custom theme, at which point that would stay as custom theme until you save and name it.
I mean that, when clicking on them, the "Custom" theme may appear, depending on the theme, but is not automatically selected. With "Numix", it does not appear at all when reopening the "Appearance" window.
I would be grateful if you could confirm that, on your system, at least one of those 3 do not automatically jump to the "Custom" theme on the first click.
Could you try with theme "Numix"? Its package "numix-gtk-theme" is in the standard repository, it installs quickly, and no reboot is necessary.
I am looking for confirmation that the behaviour is different with "Numix":
a) When you click on "Numix" the first time (in the "Appearance Preferences" window), it does not select "Custom" instead.
b) When you close and reopen the "Apparence Preferences" window, and "Numix" is selected, then theme "Custom" does not appear as the first entry in the theme list.
I upgraded my Ubuntu MATE a few times, and tweaked things here and there, and I am worried that the behaviour on my system may be different to other installations.
Just a thought, Numix is a third party theme which developement has stopped and most likely doesn't have features available for UM to fully integrate. Lot has changed in 6 years.
I am not advocating for Numix. If you follow the first link I posted in this thread, you will see that I am having colour problems with Emacs (which should use GTK3) and with Tk applications like Git Gui. All that with supposedly modern and properly-maintained Ubuntu MATE, and its default Yaru theme. The Tk problems have been there for years, apparently.
I haven't found yet a good overview about themes and how to troubleshoot them. I noticed that some themes behave differently in some respects, and I wanted to collect more information about it. Hopefully, that will help me pin-point the problem.
