Oscar,

If you are interested in building web applications, knowing HTML/CSS is a must.

I am, like you, a computer scientist and I'm happy I have spent some time in the trenches of HTML and CSS. Sure, you want to do database stuff, and need to do a lot of server-side programming too. Many people see these things (HTML versus serverside programming) as complete opposites. But with the advent of web applications (and even more so with HTML5) this line has started to blur.

So I say: learn both