Add interface to allow modification/customization of CSS for popups

[cloudtracer]

trafficstars

Make CSS customizable for all page rendered popup items. Mostly for debugging/patching the CSS on the fly for sites that do override the ThreatPinch CSS.

This could also allow users to customize the look of their popups. Perhaps we can even distribute themes in the future.

[cloudtracer]

This might be handy for troubleshooting cross browser issues.