Thanks.
After considerable hunting around I managed to get the menu that included 'look and layout' to show up -- it wasn't visible at all at first, but while making notes for you on what I could see instead (which didn't seem to include any sidebars) I noticed a double left arrow up near the top of the forum profile page, clicked that, and that sidebar appeared.
Once I got into 'look and layout' the themes told me I was in the board default, but in the night variant. I tried previewing the day variant and that worked OK on the preview, but when I tried to set it to actually use the day variant it kept switching me back to the night variant. I scrolled down the page and looked at some of the others (the 'gray' looked to me nothing like the version I kept getting stuck with -- were you maybe looking at a night variant of that?) and at the bottom of the page there was SSSS Variant which was also set to dark. I set it to light and clicked to use that, and it's OK now.
I also have no idea how I could have done that by accident. I wasn't on my profile page at all when it happened. The board default version says "your theme will change along with the administrators' settings and the board you are viewing" -- is it possible an administrator did something that caused this?
In any case, it seems to be all right now. Thanks again.