How do I remove the dark background from the menus?

lomenus

If I switch Skia on or off (it depends on what I did last time) it changes - until the next time I open a Libre Office app…

Maybe you could file a bug at bugzilla, but you need to put much moreinformation on version of LibreOffice and your OS:

https://bugs.documentfoundation.org/enter_bug.cgi?product=LibreOffice&bug_status=UNCONFIRMED&bug_severity=normal

Maybe you should read this to improve your question:

… so what? It might be inconvenient for you, but what do you expect from other fellow users on this user-to-user Ask site?

Give up. The internet is not a nice place.