How can menu toolbar background color be changed?
Tried to find from here without success:
Solid colours are defined in Prefs -> Display -> Colors and fonts
Also when clicking "This PC" the background is white.
Trying to change white to dark.
Looks like you need to turn off Preferences / Toolbars / Appearance / Use Office 2003-style for toolbars.
Either:
- turn Preferences / Folders / Virtual Folders / Native display of 'Computer' back on, so Opus handles the folder instead of the shell
or:
- you could try turning on Preferences / Display / Options / Enable background images in virtual folders (but it probably won't work well with dark backgrounds, as the shell will still draw using a dark font, unless the system-wide themes are changed).
@Leo, thank you, the toolbar tip did the trick 
Is there any workaround to have a dark background with light colored text in the "This PC" view?
Turn Native display of ‘Computer back on and it should use your chosen colors, similar to a normal folder.
It only doesn't use your color choices if the you tell Opus to delegate that folder to the shell (File Explorer, more or less) instead of having Opus handle display of it itself.
Okay, didn't get it to work, but it's cool.