Adapta GTK and Flat Remix Icon themes are most popular themes for your linux system. Adaptive Gtk+ theme is based on Material Design and it uses Noto sans and Roboto fonts. Flat remix icon theme is derived from Ultra-Flat-Icons, Paper, EvoPop and Numix-Circle icon themes.
Install Adapta GTK Theme on ubuntu:
sudo apt-add-repository ppa:tista/adapta
Then type your ubuntu password. And hit enter key confirm the installation.
Then type the below command and hit enter.
sudo apt-get update
Then run the below command in terminal to install Adapta GTK Theme on Ubuntu.
sudo apt-get install adapta-gtk-theme
Type Y and hit enter to confirm the Adapta GTK Theme installation.
To apply this GTK theme use Unity Tweak Tool.
Open Unity Tweak tool and click the themes menu and apply the theme.
Install Flat Remix Icon Theme on Ubuntu:
And unzip the downloaded file. then move “Flat Remix” folder to “.icons” folder in your home directory.
Note: .icons is a hidden folder.
To apply this icon theme use Unity Tweak Tool.
Open Unity Tweak tool and click the icons menu and apply the theme.