Install Adwaita shell theme in Ubuntu 18.10

1. Create a folder "myShellTheme" or any name as you wish in /usr/share/themes/

2. Create subfolder in "myShellTheme" named "gnome-shell"

   

3. Copy the required theme file

   sudo cp /usr/share/gnome-shell/theme/gnome-shell.css /usr/share/themes/myShellTheme/gnome-shell/
   

   

4. Close all the files, Open Gnome-Tweaks and Select "myShellTheme" under User Themes

   

You can load the vanilla GNOME shell theme (Adwaita) from your home directory. To do that follow the steps below.

1. Create a directory, say MyTheme either in ~/.themes or in ~/.local/share/themes. (You may create ~/.themes or ~/.local/share/themes if they don't exist.)
2. Create another directory inside MyTheme called gnome-shell.
3. Create an empty text file in this gnome-shell directory and name it gnome-shell.css.

4. Open this gnome-shell.css in a text editor and add the following line

   @import url("/usr/share/gnome-shell/theme/gnome-shell.css");
   

   Then save the file.

5. Launch (GNOME) Tweaks, go to Appearance section, and click the drop-down box next to "Shell" theme. MyTheme should appear in the drop-down list (assuming you've the 'User Themes' extension installed and enabled). Select it.
   (You may have to re-login or restart GNOME shell to see the effect)