Oh.. What you are facing is a bug that's been reopened on KDE.
https://bugs.kde.org/show_bug.cgi?id=459468
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.