hi, in oxygen theme, there is an option that allows changing the gradient for the oxygen window decoration:
It is under Configure Decoration - Fine Tuning - Background Style.
Default this is "follow style hint", but if set to "Solid Color" the gradients below (for the menu, toolbar) are still there, so it does not look nice.
So regarding the "hint style", is there possibly an other option somehwere else that allows to turn off the highlight gradient overall for oxygen?

Unfortunately in this case there is no dedicated option in the Oxygen style as there is in the Oxygen window decoration unfortunately.
However you may be able to get it to disable the gradient by lowering the "Graphical effects" setting in System Settings > Application Appearance > Style > Fine Tuning. I'm not sure which setting (if any) will disable it however.

Adjusting the slider in System Settings -> Application Appearance -> Colors -> Options does affect the background gradient (as well as the contrast of a few other elements)