Transparency is easy. A font is really just a transparent background with what you want to be part of the font be black is probably the best way to think about it. So the text you are reading here, the black is the font, the white background is transparent so if you made the <div> containing text have background color as red, everywhere the red is is a transparency in the font. If you use iconCls : 'home' the windows are a transparent section where the black just wasn't put.
Gradient I haven't tried. Doing a quick google search there are some work arounds if the CSS color rule doesn't accepts gradient... I've personally never tried.
Thanks. Actually I meant to say opacity, or partial transparency. For example, in the image I linked to, two of the squares are partially transparent, so you can see some of the background behind them.