Welcome back to HTML/CSS - and, of course, the forum!
W3Schools has a pretty good HTML/CSS/JS tutorial and reference site that will get you back up to speed with the latest changes. Here's a link to their CSS tutorial http://www.w3schools.com/css/default.asp
One thing to keep in mind - while HTML5 and CSS3 are out and ePub 3 is a standard, there are very few devices that support it fully (yet). Most ePubs are written to the ePub 2 standard which only supports CSS2. Sigil currently only produces valid ePub 2 - with plans to incorporate ePub3 in the (near??) future.
That might allow you to focus your studies!