You could try adding:
table.category {
width: 100%;
}
in your light3.css
(Should you change style/colour - you will need to put that in the new styles css as well)
P.S.
Your remark about browser dependency. Not sure about that. I am working on a Mac and all browser I use have the compressed look. Safari (and thus Google Chrome) - and FireFox have a compressed table.
Then on a Windows machine, IE7 has a compressed table including FireFox as well.
So here we have the major browser for at least 95% of the market showing the compressed tables.