Well I'm on the SMF theme and I see the button. It doesn't really matter, though, I guess. Highlighting at the same color as the text works fine. (Yay, I came up with something useful! And it's my original creation!)
Ah... it appears that you can assign permissions for viewing and posting spoilers to (groups) of people. Being an admin, I didn't notice that others couldn't use them. It should be fixed now.
Logged
“When Juffo-Wup is complete when at last there is no Void, no Non when the Creators return then we can finally rest.”