@syntax
On second thought you may not need to break #semanticHTML if your webpage container is after the #checkboxHack?

So in #SASS it might be (very roughly):
.themeSwitcher[:checked] + #webpageContainer {
// test for what
// @aymm
// mentioned and have styles
// accordingly?
}

Do a small test first, perhaps?

@syntax
Great discussion here.

Break all #html semantics and put your entire webpage inside a #checkboxHack. Combine with what @aymm said to invert the default and the button operation. :)

Partly joking just because we'd like to see this attempted (there's possibly a #semanticHTML way to do it).