Source (required if based on other people's work): https://github.com/infnty/adapta-gtk-theme-colorpack/
Builds of adapta-gtk-theme, a flat material design inspired theme, for a variety of colors from material design palette rather than just cyan. A couple tweaks to increase contrast were made, other than that it's all based on vanilla upstream code.
Following color variants are included, with 4 themes for each: standard light theme, Nokto - dark theme, Eta - compact, Nokto-Eta - dark and compact:
Installation: unpack tarball into ~/.themes/ to install for current user only, or into /usr/share/themes/ or install .deb to install system-wide.
Roboto or Noto Sans is recommended as a UI font for this theme.