That's a good question. Currently, we can only customize the design and splash on some colors for invoice, estimate and sales receipt. This is not applicable to statements.
You have a few options if you'd like to modify your statements. Here's how:
Go to the Settings ⚙️ menu and select Account and Settings.
Choose the Sales tab and scroll to the bottom to Statements.
Select Edit ✏️.
You can choose either List each transaction as a single line or List each transaction including all detail lines.
Turn Show aging table at bottom of statement On or Off by selecting the checkbox.