BlueBreeze

To make the top bar fully transparent in Gnome’s arguably most popular theme, Arc, It would only require a couple of changes in the gnome shell css:

/usr/share/themes/Arc/gnome-shell/gnome-shell.css change this

#panel {
  font-weight: bold;
  height: 2.1em;
  min-height: 27px;
  background-gradient-direction: none;
  background-color: transparent;
  border-bottom-width: 0;
  border-image: url("common-assets/panel/panel.svg") 1 1 1 1; }

to

#panel {
  font-weight: bold;
  height: 2.1em;
  min-height: 27px;
  background-gradient-direction: none;
  background-color: transparent;
  border-bottom-width: 0;
  border-image: none; }
#panel.solid {
  border-image: url("common-assets/panel/panel.svg") 1 1 1 1; }

and this

#panel:overview {
  border-image: url("common-assets/panel/panel-overview.svg") 1 1 1 1; }

to

#panel:overview {
  border-image: none; }

Then restart the gnome shell, or simply press F2 then type in rt to refresh the application.

Another approach:

Open Terminal and run the following command first to allow custom alpha values for Ubuntu dock

gsettings set org.gnome.shell.extensions.dash-to-dock customize-alphas true

Then run the following to set the minimum alpha value (i.e. when a window is not maximised or touching the top-bar or dock)

gsettings set org.gnome.shell.extensions.dash-to-dock min-alpha 0

Finally, if you want, run the following to set the maximum alpha value (i.e. when a window is maximised)

gsettings set org.gnome.shell.extensions.dash-to-dock max-alpha 0

Then log out and log in again.