When using ext-sandbox.css (as in http://dev.sencha.com/deploy/ext-4.0...xt-sandbox.css), most of the style definitions are scoped:

.x4-reset html,.x4-reset body,.x4-reset div,.x4-reset dl,.x4-reset dt,.x4-reset dd,.x4-reset ul,.x4-reset ol,.x4-reset li,.x4-reset h1,.x4-reset h2,.x4-reset h3,.x4-reset h4,.x4-reset h5,.x4-reset h6,.x4-reset pre,.x4-reset code,.x4-reset form,.x4-reset fieldset,.x4-reset legend,.x4-reset input,.x4-reset textarea,.x4-reset p,.x4-reset blockquote,.x4-reset th,.x4-reset td{margin:0;padding:0}
However, there is in fact a one (and only one, as far as i can tell) definition that is not scoped. This is the 15th definition in the file:

    color:black;font-size:12px;font-family:tahoma, arial, verdana, sans-serif
This one, of course, wrecks havoc on our solution :-)