Invoice themes determine how the invoice PDF looks.
Only the invoice themes that you create can be edited, these are listed under the My Themes heading on the invoice themes menu.
Navigate Settings > Configure system > Invoice themes.
Click the 'Options' icon beside the title of the invoice theme that needs to be edited and select the 'Edit' option.
Different elements of the theme can be edited by changing the information in the Theme settings, Labels and Styling areas, as well as looking in the Preferences tab.
Preview the invoice and click Save theme to save the changes.