Taken From ASP.NET Forums
Microsoft provides a style sheet named
ReportingServices.css that is used by
SQL Server Reporting Services. For my
local workstation installation this
style sheet is found in the c:\Program
Files\Microsoft SQL
Server\MSSQL.2\Reporting
Services\ReportManager\Styles folder.
The classes that are referenced by
different html elements are predefined
in this style sheet. You style your
reports by editing this style sheet.
There is no link to this style sheet
in a report's html. There is,
however, an inline style section in
the html rendered for each report. I
believe that Microsoft creates the
inline style section based on the
contents of ReportingServices.css.
Doing it this way (rather than
allowing you to link in your own style
sheet) allows the report rendering
engine to have the knowledge it needs
to control page breaks, column
wrapping, and other formatting issues.