SUCCESS!
I just created the black text on black background problem today when commenting on youtube. Text appeared black-on-black but reverted to normal white text on black background when the youtube comment was posted. The link mentioned above (and posted below) fixed it as far as I can tell.
edge://flags/#enable-force-dark
What caused this using Windows 10? I do not exactly know, but I am fairly positive it was because the heavy cuff on my winter jacket was pressing/holding along one of the keyboard's keys along the row of keys to the right of the space bar (while pressing another keyboard key).
I heard a single high pitched ring (sort of like Super Mario snatching a gold coin?) and a dialog box popped up. I didn't read it and cleared it with the "X" in the top right corner of the box with the mouse.
Should I have any more probs, I will probably just reset Edge to its factory default settings.