I do not think you need to deny access (although I think that if the webpage doesn't need colours, it shouldn't set any at all, instead using the user's prefere

I do not think you need to deny access (although I think that if the webpage doesn't need colours, it shouldn't set any at all, instead using the user's preferences), but (as defanor mentioned) you can apply CSS (although it doesn't have to be global; I generally apply user CSS individually instead, since each webpage needs a different CSS to be fixed; usually webpages that don't already have CSS don't need any CSS to fix them, I find, though)