[qualiteam]

At the moment the only way to do this is creating a simple custom module or adding a custom CSS file via Custom Skin module. The classes that render invoices and the invoice page are \XLite\View\Invoice and \XLite\View\InvoicePage (however, you may need to decorate a few more). You should decorate these classes from your module and add custom CSS files via the getCSSFiles() method.

Also, as far as I know, one of the next 5.3 versions will add a function to edit invoice templates.