As shown in the project file, if a background color is set the Dark Theme acts perfectly fine.

Starting or switching back to the Light version of the theme, will set background to white.

If you can see the same behaviour happens to Qtcreator (DarkMode) 

Update: https://bugreports.qt.io/browse/QTBUG-72584

This bugReport shows the same problem. In the end of it, a work around involving the use of background: instead of background-color: is not solving. I already tried it before pushing this bug report.