MMWI themes are built with RazorEngine, Razor MVC Views, jQuery and knockoutjs.
You can make your own theme for MMWI.
Or modify an existing one.
Each MMWI theme must be in it's own directory inside the Themes folder in the install dir.
The folder name will be used for theme name inside MMWI.
The Themes folder there is also a "_shared" folder which contains files available to all themes. Reference these files from your theme code like this:
That is: you reference a virtual folder called "shared" instead of "_shared".
All other file references are automatically mapped to the active theme directory.
Things to note
Other than that is should be pretty straight forward.
The easiest way of getting started is by looking into one of the existing ones and playing around with it.
When developing themes make sure the tick the "Disable theme caching" option in the Windows app.
Theme files are all cached in memory for performance. Ticking the option triggers a cache refresh whenever a theme file changes.