After updating to 22.04 LTS themes no longer save on a restart. Instead of using the ones I have selected it just reverts to yaru light. Any help would be appreciated. Additionally where is the picom config file because after upgrading any changes to the config file make zero effect. Thanks.
Perhaps related to your problem with themes, I noticed that my changes to 'window borders' were resetting to Yaru. Apparently, Ubuntu MATE 22.04 ignores metacity themes, after logout or reboot, if there isn't a version-1 file present. I solved this by putting a blank file named metacity-theme-1.xml next to the metacity-theme-2.xml file, in the metacity-1 directory of the theme that I had added.
If anyone wonders (because I did) the empty metacity-theme-1.xml have to be created inside metacity-1 directory which is located inside your theme directories. Themes directories in Ubuntu Mate are located in /usr/share/themes/ so for my theme I had to simply do:
sudo touch /usr/share/themes/Numix/metacity-1/metacity-theme-1.xml