From Help Wiki
Customize the CSS of your theme to change colors, fonts, etc.
You can write custom CSS (Cascading Style Sheets) to override the theme's CSS.
- Activate the Simple Custom CSS plugin in your site
- Go Appearance > Edit CSS and begin creating your own style sheet to override the built-in theme's style sheet
- You'll need to learn how your theme's HTML and CSS is structured, primarily what are the names of the IDs and Classes used so you can override them in your style sheet. Some great tools for this are:
- Firefox Firebug
- Safari's Develop Menu
- Firebug for Chrome
- To learn CSS basics try w3schools.com