Developing module/small webpage to style in other sites with security

See more:

C#

Javascript

.NET

HTML5

.NET4.5

I have a request to develop a interface which must be used in different websites. They will style this interface with there own css (only color and font)
Before they can connect, i'll must be able to receive and check some credentials or token or url

I'm free to start in any language/script I choose, but like to avoid version conflicts(scripts).
My knowledge is .net / javascript, jquery, html, css and xml.

this interface will be appear on the page like an iframe presentation
Security is important
Css file on the website where it's displayed(can be same path for everybody).
Only a few simple option will be needed.

Anybody experiences with create something like this?
I'll be great full if you'll will help me.