CODE EXAMPLE FOR SHELL

center dock icons ubuntu

First Install dconf-editor

$ sudo apt install dconf-editor

Then 

$ gsettings set org.gnome.shell.extensions.dash-to-dock extend-height false
$ gsettings set org.gnome.shell.extensions.dash-to-dock dock-position BOTTOM
$ gsettings set org.gnome.shell.extensions.dash-to-dock transparency-mode FIXED
$ gsettings set org.gnome.shell.extensions.dash-to-dock dash-max-icon-size 64
$ gsettings set org.gnome.shell.extensions.dash-to-dock unity-backlit-items true