That all sounds right. Just a couple things to remember - css is case sensitive. Are you using the redmond theme in your app? I think the default is ui-lightnes (not sure).
Do you have the firebug plugin for Fire fox? You can see if your styles are getting reset by using it. You can see the over-ridden styles will have a line through them.
You don't need to use Gzip all to include your css file just include it in the file list. Also, unless you actually add it to the all.css file, make sure the "Is Included in ALL.CSS" box is unchecked.
As far as changing one field when another changes...
In the 'trigger' field, on the client side tab, check "send new value to server" - click the Server Code button and add your code to set the css selector. Something like
Case triggervalue
of '1'
loc:css='green-bg'
of '2'
loc:css='red-bg'
end
Add the the 'changed' field name to the reset list.
In the 'changed' field, on the CSS tab, put loc:css in as the EntryDiv Class or the Entry class.
That should do it.
Bruce may see something I missed here (he usually does <g>)
HTH
Chris