Using Template CSS Editor, you can edit online selected template's template_css.css file that holds the CSS classes. In order to be able to save your changes, the template_css.css file must be writable. Its status is displayed next to the file name, above the editor. Be sure that is set to Writable.
Checking the "Make unwriteable after saving" option, will change file's permissions in order to make it unwritable.
Save: Click the "Save" icon to save changes.
Cancel: If you want to discard changes click "Cancel" icon.
Help: This Help Screen.