Make the WordPress Editor Look Like Your Web Site
Step Two: Tell WordPress to look for the editor-style.css file
Just adding the editor-style.css file to your theme folder will not make the change on its own. You need to add the following code (again
stolen borrowed from Twenty-Eleven) to your functions.php file.
// Styling the visual editor with editor-style.css to match the theme style.
Save your changes and reload your editor and you’re done! In two easy steps, your Visual Editor should now be pulling in the styling form the front end of your web site.
Nifty, huh? And now truly WYSIWYG.
The best part? No more having to explain why the font doesn’t look the same!