There are many flat themes available for Linux desktops, you may have favorite one also. Plano another flat theme specially designed for Gnome and Xfce desktops. It is compatible with Gtk 3.24/3.22/3.20 versions, 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.
Available for Ubuntu 17.04 Zesty/17.10 Artful/and other Ubuntu based distributions
To install Plano GTK theme in Ubuntu/Linux Mint open Terminal (Press Ctrl+Alt+T) and copy the following commands in the Terminal:
Plano Another Flat Theme For Gnome And Xfce Desktop