4
Shijk:
You can't find it because it's not defined. In the default stylesheet the selector fieldset is not included. However, if you see the files theme_blockcenter_c.html, theme_blockcenter_l.html, and theme_blockcenter_r.html (that are also part of the theme, of course), you'll see the selector invoked, though not defined.
So the solution is to add it to the stylesheet just as I suggested.
Cheers.