Yes, that's definitely doable. However, you'll need to do that on the Xero side.
This is how it works:
In Xero, click on the name of your Organisation and go to Settings —> Invoice settings. Here, change the theme. You can add a logo as well as change some information and the layout.
Save your changes and sync this data back to ApprovalMax.
To do so, go to either the approval or Requester matrix in any workflow of your Organisation. Alternatively, in the main menu, select your Organisation and open Organisation settings and connections under Workflows and settings.
Click on the icon and then on SYNC NOW next to Themes.
Please note: If you need more complex changes to how your Purchase Orders display, please use Xero advanced invoice templates.