You sure can!
You may have already seen this little video about how to customize the Standard Invoice Template through the Xero administration.
If you want to create your own Custom Branding Theme though you’re going to need to use Word. Xero TV has published a great video on how to make your own “Advanced” templates. You can view the video here.
Best of luck! If it all gets too hard you can always Order a Custom Theme from us too.