It is possible, but it’s done a bit differently and requires a Power User Subscription. If you have a L or XL account, it’s likely you already have this.
If you go to Account Settings >> Design Customisation >> Advanced CSS & HTML Customisation, you can edit the CSS for both Purchase Orders and Client Statements here.
Are there any particular changes you were looking to make? More than happy to help point you in the right direction if needed.