The intuitive way would be for Stickies to have a Preferences window where you specify your defaults. Stickies prides itself on being a nimble app however, so it doesn’t have a Preferences window. Instead, you select a sticky that you’ve customized to your heart’s content, go to Window, and select “Use as Default.”
I’ve been using a Mac for more than a decade and I never knew this until today. I’m delighted to have finally changed my default Stickies background from yellow to blue.