I upgraded from Ubuntu 19.10 to 20.04 and have not found a solution to these desktop bugs and don't know how to report them.
See images below, icons for all system applications are missing, including minimize, maximize, and close icons for all applications, period. The top right menu at first was invisible but now only shows the airplane mode icon and clicking the background right below it activates it, as if the menu had the wrong dimensions. Full screen applications can't be closed with the close window icon because you'll be clicking the menu instead.
I have reinstalled
gnome-session, and other packages to no avail.
Running this command
gsettings get org.gnome.desktop.interface icon-theme gives me:
GLib-GIO-Message: 12:10:31.048: Using the 'memory' GSettings backend. Your settings will not be saved or shared with other applications. (process:5770): GLib-GIO-WARNING **: 12:10:31.048: unknown schema extension 'd' (process:5770): GLib-GIO-WARNING **: 12:10:31.048: unknown schema extension 'd' 'Adwaita'