SSRS does not support custom CSS. See here. I think I read somewhere that you can hack the stylesheet for your report server instance to make things look different, but that mainly applies to the parameter pane, I think.

You can create your own report templates so you can have a standard look or looks, see here.

Adam Aspin has also written a set of articles for SSC on how he applied styles to SSRS reports. You can find the list of articles here[/url].