If you want to make your own theme, please check the Documentation of "Static blog/website generator" section.
How to use
It's very easy, just copy the theme folder to your MWeb themes folder. MWeb themes folder is in
If you use git, you can also use bellow command:
cd ~/Library/Containers/com.coderforart.MWeb/Data/Documents/themes/SiteThemes git clone 'git repo url'