Both accounting softwares, Xero and Quickbooks, allow for some sort of customization. Both provide you with some templates that you can take and customise further, as well as an option to upload your own template.
The terminology used to describe these templates differs - Xero calls them branding themes whereas Quickbooks refers to them as form styles. The idea behind them, however, is exactly the same - a way for you to personalise your documents by adding your logo, brand colours, headings, watermarks or extra messages.
The list of documents that the two accounting softwares allow you to customize are almost the same.
Xero lets the user customize Invoices, Statements, Credit Notes, Quotes, Purchase Orders, Receipts, and Remittance Advices.
In QuickBooks Desktop, you can create custom templates for forms like Invoices, Sales Receipts, Estimates, Statements, and Purchase Orders.