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I accidentally clicked on the tiny white square next to "Color" in the dropdown menu that opens when you click on "Background" in the very lower left of the timeline. There are 3 options in that dropdown menu: "None"
"Color"
"Check"

And it turned the background color to a dark grey. Does anyone know how I can get the white background color back again?

make sure that you have a white color directly available in the colorpicker (or anywhere else in your interface for that matter)

then go to the background menu and
find the button.

this is a bit like searching for a lightswitch in the dark... its there but you can't see it.
if you hover slowly a notification will pop up.
but if you know where to click you can do that directly.

left click on the bottom left one

you will get a cursor with a question mark that lets you click on any color in your interface... click on something white and you have your white BG back

Yes, that fixed it. Thanks so much Peter!