Should I add my own CSS to my tumblelog?

Google is your friend. Search for a beginners css tutorial

CSS stands for C ascading S tyle S heet. This code is usually referenced by your blog as a standalone document, so the formating becomes a modular add-on to the rest of the page.

The purpose of this code is to redefine how certain portions of you blog will look. For instance, the question box on this site has a yellow background with an orange title-bar and red lettering on the heading. This is handled in the stylesheet. The HTML code for this page only had to say that there was a question area and a heading within that question area; the stylesheet formatted it.

The usefulness lies in changing your format quickly, as a stylesheet on this page would produce the exact same content (HTML would not change); but it could, perhaps have a green background with blue headings in the question area.

You may try to recycle another stylesheet from someone else, if you see a look that you like. Get permission first, though.

If you really want to get into writing CSS,

http://w3schools.com is of great help, as is the book,

HTML, XHTML & CSS, (ISBN 0–321-43084–0).

Worked great to teach me. Book goes over a lot of material rather quickly. It has many examples, which helped me learn this very quickly.