How change background color of multiple slides simultaneously

LO on Ubuntu 20.10 here. I want to change the background color of a number of slides. I tried the following:

  • In the left “slides” pane, I selected the slides, then changed the background using the (right) properties pane. That only changes the background of the current slide, despite the fact that multiple slides are selected.
  • Try the same in the “Slide sorter view”
  • Tried both options above, but then using the menu “Slide - Slide Properties” to change the background.

Current case is that imported slides have yellow background. The Slide Master has the background set to “None”. Imported slides have been explicitly assigned the “Default” Slide design in “Slide - Change Slide master”.

Hard to believe that, in 2020, one cannot change properties of a few objects at the same time. I hope I have overseen something. The (to me) most obvious approach did not work.

Work with Master Slides.

Doesn’t work. Applying a different master slide does not reset the background.