Several times I have talked about the importance of being familiar with CSS, especially as you make tweaks to your design.
A great resource for learning the basics of CSS is the w3schools.com tutorial.
Another great resource is the CSS 3 Cheat Sheet — a printable document that allows you to see all the possible properties at quick glance.
(P.S. While you're at it, you might as well grab the HTML Cheat Sheet too!)
Here at BloggingWithAmy you'll find tips, tools & tutorials about blogging and websites. In plain English. Want to 
I designed my new maths blog using the atahualpa theme after reading your step by step guide to setting up a blog! The cheat sheet would have saved me a lot of time and stress, but, thanks to all the other pieces of helpful advice from your blog, I can look back now and say that I "designed" my own blog!