The mtheme is a Beamer theme with minimal visual noise. The core design principles of the theme were described in a blog post. Not convinced? Have a look at the demo slides.
For further information take a look at the manual.
The theme itself is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License. This means that if you change the theme and re-distribute it, you must retain the copyright notice header and license it under the same CC-BY-SA license. This does not affect the presentation that you create with the theme.