The Divi Theme's "email optin" module (formally called the "subscribe" module) allows you to add a form to your pages which users can use to join your mailing lists. A tutorial by Geno showed how to use CSS to change the layout of the module's signup form into a horizontal format. Unfortunately changes in Divi's implementation of the module have stopped that code (and the Divi Booster feature based on it) from working. I recently revisited this implementation and came up with a new, simplified, way to make the email optin module's form horizontal via CSS.
Here's the CSS code:
Related Post: Adding CSS to the Divi Theme