Change the Style of the Default WordPress Post Editor
Did you know that WordPress 3.0 allows themes to style the visual editor? This can be useful if you want to make the editor look more like the front-end for your client and make it more like a custom-built CMS.
Add this to your theme’s functions.php file:
Now you can create a custom CSS file that will change the style of default post editor. The function above tells WordPress to look for the custom editor stylesheet in the root folder of your theme. If you want to work from an example, grab the editor-style.css file from the TwentyTen theme and change it to suit your needs.