If i understand your question correctly:
Go to your WordPress installation public_html folder, open up wp-config.php and past this into it on its own line at the bottom:
define('DISALLOW_FILE_EDIT', false);
that will then activate the “Editor” tab on the flyout menu in the themes “Appearance” link.
Be careful WP has no option (well it may do now but i highly doubt it) to reverse any changes or has syntax highlighting, your best option is to ftp the themes style.css to your workstation, copy it, then work on it remotely and upload when done, the copy is your insurance policy if it all goes wrong you can just reupload the original version.
when done either remove the line you pasted into the config file or change false to true to hide the editor again.