Adding Css To post in Custom Dashboard widget

I have a custom dashboard widget that is created with a custom post type... I embedded the Facebook like box but it is not fluid with the screen

I found this fix to make the like box fluid so it could stay in the module when the screen adapts in and out but not sure where to put this css to make it work.. any ideas

.fb-comments, .fb-comments iframe[style] {width: 100% !important;}

<iframe src="//" scrolling="no" frameborder="0" style="border:none; overflow:hidden; width:600px; height:290px;" allowTransparency="true"></iframe>