I've upgraded to Ubuntu Desktop 20.04. (sudo do-release-upgrade after regular sudo apt update and sudo apt upgrade)
Several things in the appearance now seem off. The terminal previously had a maroon background colour, now it's plain white. The terminal scroll bar on the right is badly positioned. See screenshot here.
The icons used don't appear as expected. I'm expecting something like this. Instead I see this. Look at the icons on the left here.
The Language Support icon also appears as a white silhouette.
Changing theme variant under Settings > Appearance to "dark" has no effect.
Something in gnome needs resetting?
More info:
I just installed Gnome Tweaks, "Applications" was originally set to Adwaita. See here. Screenshot doesn't enable capture of picklist options, but the options are Adwaita (default), Ambience, HighContract, HighContrastInverse, Radience. The icons seem to be messed up; For example, there's no difference between HiColor, DMZ-White, DMZ-Black.
Options for Icons are Adwaita (default), DMZ-Black, DMZ-White, Gnome, HiColor, HighContrast, Humanity, Humanity-Dark, LoginIcons, Ubuntu-mono-dark, Ubuntu-mono-light.
The default "Yaru" theme for 20.04 is not listed, but is present under /usr/share/themes/
Resetting to defaults causes all Appearance "Themes" options to go blank.
XDG_SESSION_DESKTOP:
$ echo $XDG_SESSION_DESKTOP
ubuntu
echo $XDG_SESSION_DESKTOP. Please edit your question and clarify.