Include Materia theme
Adapta is not maintained anymore and Materia is a popular flat, material alternative, actively maintained with superb coverage of applications and three theme variants. There is also a high quality kvantum version for qt maintained by Papirus (https://github.com/PapirusDevelopmentTeam). It's a really good theme to have by default.
Also it's the default theme for Ubuntu Studio, so it fits the goal of this package.
Edited by Carlos Pita