Customize css

How can i remove the border of widget or how can i customize the class inside the piwik iframe?
My client has registered in piwik from this domain—>
I feel it’s all encrypted…Here is the file from where the css is coming from—> http://********.com/piwik/index.php?module=Proxy&action=getCss&cb=bcae6cc2903ee97ae7d378859bce6bd0 and I am not able to edit or override any of the css from my css(theme).

I think this link will be helpful
customize css