This example shows how to change the designer 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 25 ready themes, and you can also adjust the CSS file as you require. For example, apply the Office2013 DarkGray Green theme to the designer:
<!-- Report designer Office2013 DarkGray Green style -->
<link href="/../css/" rel="stylesheet">

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

Change the Report Designer Theme