We offer complete Telligent Theme Services which you can learn more about here: http://www.coutosolutions.com/TelligentCommunityServerServices/Branding/Overview.aspx
There are lots of ways to theme Telligent with some being better then others. If you are looking at a CSS theme that can be the easiest way for most designers. If you are going to need custom HTML, then you are looking at doing custom widgets or widget modifications using widget studio.
When we build theme's we usually do a commbination of build new default widgets and CSS that will become your new default theme making your custom theme developed by us your base theme.
If you need any help, please feel free to reach out to us. Our company is solely focused on Telligent work and we have lots of examples to share with you if you are interested.