I'm a long time backend java developer who has always struggled with frontend tech. The situation today is actually pretty nice; with bootstrap (or other) you can get a "decent" looking site with very little effort (ie better than raw html). And if you just want to work with API, there are a few jquery ui base widget libraries around[1] I've used in SPA's before. Good luck!

[1] http://www.jeasyui.com/