Changing the background of all slides at once in LibreOffice Impress

In LibreOffice 5, if I wanted to change the background of all slides at once, I could go to “Slide > Properties”, change the color, and then apply it to all slides.

In LibreOffice 6, I can’t find an option to apply it to all slides at once; I only seem to be able to apply it to a single slide at a time, which as you can imagine is painful.

Now, I know the right way to do this is with master slides. However, I’ve got a bunch of slides I’ve assembled from different presentations, and a lot of them have had their background color set individually (i.e. they aren’t getting it from the master slide). So, changing the master slide doesn’t change them all at once; I’ve also got a bunch of different master slides as a result.

Where can I find the option in LibreOffice to apply slide property changes to all slides, not just the current slide?

As a related question, how do I remove individual customizations to slides and force them to have their properties revert to their master slide?

(Actually, I can’t even find in the menus where you set which master slide to use for a given slide. Is that bulk-settable, so you can change all the slides at once?)

(Version upgrades are always painful.)

Use the Styles. The “Presentation Style” named “Background” is the default style for the backgrounds of all Slides. just modify the relevant property of this Style, then the background colors of all slides will be changed. It works in that case only, when the bakground colors was not modified manually. If they was modified manually, then you must reset this properties before.

Impress styles.odp

That’s fine for starting and moving forward. The problem is if I have a pre-existing presentaton, or slides culled from a bunch of different presentations, that have their backgrounds set. I want to be able to bulk change them (or bulk remove them, so that they will adhere to a style). How do I do that? It was trivial in LibreOffice 5, but so far I haven’t figured out how to do it other than one-by-one in LibreOffice 6.