Not sure if this should go here or not, so go ahead and move it if needed.
It took me a while to figure this out, so I thought I'd share. Perhaps something like this has been posted before, but I haven't seen anything like it, which means maybe other EHellions haven't, either. I think this will work as an effective spoiler tag for all color schemes, AND it should be readable just by highlighting (that might depend on your computer's or browser's settings, though). I have poor eyesight, and I can manage it, though I'm sure some people will still have to hit "quote" in order to read it in the text box. Anyway, do this, but change the <> to square brackets; if you don't want to type out the code, you can just use the glow and change color buttons up at the top and change them to beige and white. I chose white for the text because it's probably the hardest to read; beige still comes through when I use that for text, though it's not easy to read, either:
Here is some sample spoiler tagging.
I've seen black-on-black suggested, but on my computer, highlighting that to read doesn't work at all. White-on-beige seems to provide just enough contrast that it can be read when highlighted.