Your users probably reaches your new developed SSRS reports through some kind of portal, i.e. an Intranet based on Sharepoint, so
when designing, you of course want your reports to be a part of the existing graphical profile.
This blog post will describe how you in an easy way change the look and feel of your SSRS Parameter Banner.
1. Go to your disk where you have your SSRS instance installed
The default location of the HTMLViewer.css file:
C:Program FilesCommon FilesMicrosoft SharedWeb Server Extensions15WebServicesReportingStyles