I was a backend guy and was faced with the same dilemma. I thankfully listened to the engineer in me (at that time we had our UI work contracted out and was not shaping up well). It took a while to get a good grip of HTML, CSS and JS. They are the tripod on which today's web UI is balanced. In the end I think it was the right decision to learn it all!

Moreover if no core team member knows UI - then either get one or you become one, becoz sooner than later you will repent outsourcing it fully.