Material theme has been added in version 1.3. It does have a lot of differences with default theme:

• HTML files - some of HTML files are different, mainly because some of features from default theme aren't supported
• LESS files - here everything differs: all files are different from default theme - variables, components. Colors and default bootstrap theme are the same in all themes
• JS files - mainly are the same as in default theme, but some of them have minor changes in integrated themes

Folder and file structure is the same in all themes.

Since components and core features are identical in all themes, you can easily switch between themes - you just need to replace CSS files in your current theme with those from a theme you want to use. Also, depending on component or feature, you can also review JS files with example configurations located in global_assets/js/demo_pages/ folder - some pages contains a bit different configurations to match theme specifications: file uploaders, selects classes, colors etc. Nothing too important.