Hope the below link will help you

http://msdn.microsoft.com/en-us/library/ms345247(SQL.90).aspx

Reporting Services provides a sample style sheet Htmlviewer.css that you can use as a temple to create custom styles for the report toolbar in HTML Viewer. Htmlviewer.css is located in C:\Program Files\Microsoft SQL Server\MSSQL.3\Reporting Services\ReportManager\Styles. If you want to change the color of toolbar background, you can make a copy of the htmlviewer.css to work from and make the following modifications:

.MenuBarBkGnd

{

background-color: #ffffff;

}

If you want to apply the custom style sheet for one report, you can specify the style sheet on a Report URL, such as http://localhost/reportserver?/AdventureWorksSampleReports/Product+Line+Sales&rs:Command=Render&rctylesheet=MyStyleSheet.

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Please feel free to let me know if you are not clear or I’ve misunderstood anything.

Thanks,
Arunkumar S P