How to change the style of widget(or dashboard) embeded in my website

Widgetize the full dashboard:You can also display the full Piwik dashboard in your application or website in an IFRAME (see example).You can write like this :

When I embeded it in my website,the style of iframe is not fit,just like this iframe.png.
I only choice two widget,each widget is still small.
I want to know how to change the style of iframe embeded in my page in order to look beautiful.
Thanks !!