I can only try to help you here but most likely you would have to wait for either kde to fix it or roll back to a earlier working snapshot while you wait for kde to resolve the bug.
Let's correct one thing at a time from what you confirmed earlier in your /etc/environment file add this line at the end of the file
GTK_THEME=Sweet-Dark
save it and restart your system and check if launching any apps using their app launcher helps you use the gtk theme like before or are you still stuck. If you are able to work this one out then we can look at your cursor next.
I have been trying to look for a similar environment variable online that can help your issue but I haven't been able to find anything more useful. Since this is a bug from KDE's side I suppose the only thing we can do is either wait or go back a snapshot. Though that route too seems impossible considering you have a very recent install (the habit of reinstalling your os again and again is coming back to bite you).
I will try to look a little more but can't really promise anything more than wait for bug fix to arrive. Try to search on your own end too.