On chrome I have a bookmarklet that disables stylesheets I suppose you can easily modify it so you can use a userscript.

  javascript:var%20i=0;if(document.styleSheets.length>0){cs=!document.styleSheets[0].disabled;for(i=0;i<document.styleSheets.length;i++)%20document.styleSheets[i].disabled=cs;};void(cs=true);