Ubuntu Linux already comes with the Yaru theme. But if we want to make our desktop even more beautiful by installing another theme or icon, then we have to take the following steps:
Install Gnome Tweaks
Gnome Tweaks provides many options to customize Ubuntu desktop as we wish. In order to install it, enter the following command on the terminal:
sudo apt-get install gnome-tweaks
Now go to your home directory and create a folder named .icons and .themes. The home directory is usually at /home/<yourusername>. If you do not see the .icons and .themes folders, then click on the menu of Nautilus and click on the Show Hidden Files check box.
Now open website of gnome looks and click on gtk3 themes.
Download the theme you want from here and then extract those compressed files into the directory /home/<yourusername>/.themes
The same process is to be followed in the case of icons too. You can get the icon by clicking in Full Icon Themes. Or click on the following link:
Download the desired icon package from here and then extract those compressed files into the directory /home/<yourusername>/.icons
Now start the Gnome Tweaks
Click on the Appearance tab. Here you will see your copied themes in the applications under Themes. In the drop down of Icons section you will see your copied icons. As you select them the look and feel of your desktop will change completely.