This example shows how to change the viewer default theme. All you need to do is to replace the CSS file of the theme in the <head> section to another of the available themes. Available for more than 15 ready themes, and you can also adjust the CSS file as you require. For example, apply the Office2007 Blue theme to the viewer:
<!-- Report viewer Office2007 Blue style -->
<link href="/../css/" rel="stylesheet">

In the screenshot below you can see the result of the sample code:

Change the Report Viewer Theme