I don't recall if SSRS 2005 is able to use .css, but from my vague memory it seems no.
For SSRS 2008 or SSRS 2008 R2, there are 2 .css files you can customize:
1. customize Report Manager’s appearance by changing its Cascading Style Sheet
(\Program Files\Microsoft SQL Server\MSRS10.MSSQLSERVER\Reporting Services\
2. you can change appearance of HTML viewer (HtmlViewer.css)
(\Program Files\Microsoft SQL Server\MSRS10.MSSQLSERVER\Reporting Services\ReportServer\Styles)
Hope this helps!