Other MemberMouse customers are using MemberMouse alongside the Divi theme. You can customize the html and css of MemberMouse core pages, but you don't have to code. Although the MemberMouse core pages are provided as working examples that you can customize, they are designed to work out-of-the-box with no extra coding on your part. See:
With visual editors such as the one provided with the Divi theme, you can create multiple blocks of content in order to assist in laying out your pages. One thing to be aware of is that all [MM_Form_...] SmartTags must be included in the same visual editor content block as the [MM_Form]...[/MM_Form] SmartTag in order to work. This is something to look into if you find that a core page isn't functioning as expected. This article provides details:
We're excited to announce the release of a free layout pack created to work seamlessly with Divi and MemberMouse. The Divi Layout Pack for MemberMouse replaces your default MemberMouse Core Pages with modern, responsive Divi compatible pages that look good with any style membership site. Installing the layout pack takes only a couple of minutes, and when you’re done, you’ll be able to point, click, and edit your MemberMouse site with one of the most popular point-and-click theme builders available. Here's a link to our blog post where you can take a peek at the pack and download!