ergophobe - 8:01 pm on Mar 26, 2013 (gmt 0)
assume @swa66 meant "you" as in your website, not the user having created a stylesheet
if you set a user stylesheet, it'll override *any* CSS (even inline) that a webmaster might have added
Lots of people with accessibility issues will learn how to do this.
And though I don't have any accessibility issues, I got so perturbed at Google for making its ads indistinguishable from the organic results (at least for colorblind me), I have a user stylesheet for Firefox and Chrome that highlight ads so I can tell at a glance which is which.