Check out the 2021 Fiddlerman “White Christmas” Group Project.
Basic stuff, nothing wild. But ok if you want a quick review or maybe never took theory before. This is an "at your own pace" type of course, with no final exam or projects required.
Mildly interesting point for US musicians, this isn't a US based teaching place. So if all you've ever seen are things called quarter notes and sixteenth notes (for example), hearing them called crotchets and semiquavers might be a little new at first. But it can be good to learn terminology that you may run into when talking or playing with musicians from other places, and it is not that hard to get used to.
A good point of this course over some others I've seen is it is pretty much just basics, and it does include a bit of aural (ear) training. Not to worry, it starts you on very easy to recognize intervals (octave, 4th and 5th).
So if you're looking for a review or haven't learned basic theory before, this isn't bad for it.
It doesn't seem to want me to link directly to the course page, but the music theory course is on the front page at the moment. Right under "Canine DNA Evidence Collection" and next to "A Day In the Life of a Hairdresser". LOL
"This young wine may have a lot of tannins now, but in 5 or 10 years it is going to be spectacular, despite the fact that right now it tastes like crude oil. You know this is how it is supposed to taste at this stage of development." ~ Itzhak Perlman