There are many flat design themes available for Linux desktop, here is another one Matcha based on Arc theme. It is a flat theme with transparent elements. It has theme for Gnome Shell to go along with GTK theme. It is designed to work with most of the desktop environments including Gnome, Unity, Xfce, Mate, Cinnamon and so on. If you are using distribution other than Ubuntu/Linux Mint then download zip file directly from theme page and install it in this location "~/.themes" or "/usr/share/themes". There is also theme for Gnome Shell which can go along with its Gtk version. If you find any kind of bug or issue within this theme then report it to creator and hopefully he will fix it soon.
matcha gtk

matcha gtk

Available for Ubuntu 17.04 Zesty/17.10 Artful/and other Ubuntu based distributions
To install Matcha GTK theme in Ubuntu/Linux Mint open Terminal (Press Ctrl+Alt+T) and copy the following commands in the Terminal:

That's it
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