When changing the system theme some widgets do not adopt the palette change.

From what i can tell at least QScrollArea and its content widget and also QCheckBox text is not adjusted.

See the attachment.

The scroll area and the contents widget in "Allgemein" is static, those contentwidgets in plugins are dynamically set. 

as you can see changing the color does not affect the existing (scrollareacontent)widgets while new ones created have the correct palette. 

Also the text in checkboxes is affected.

Setting stylesheet

QScrollArea { background: transparent; }
QScrollArea > QWidget > QWidget { background: transparent; } 

At least is a workaround for the background