If you're trying to create a template for the purchase order, the option is currently unavailable. The design you see when you print the purchase order is based on the standard invoice template, but it cannot be specifically customised.
To change the style and color, you can follow these steps:
Go to the Gear > Custom form styles.
Click the Edit link of the Standard template.
In the Design section, modify the template, logo, colors, fonts, and page margins.