You can customize your envelope’s font and color by clicking on the sample text. A menu will appear on the left with two sections: Typefaces and Colors. Click on either menu to see the options available.
You won't be able to modify the actual text on the envelope from the Customize page, however. Instead, the names you enter into your add guests/recipients page will be populated there automatically for each recipient. If you leave the "Name" field blank, the name you've given your card will appear instead.