Why does Radiance have GtkLabel while Ambiance doesn't?

I was looking at the gtk-widgets.css files in Ambiance and Radiance (/usr/share/themes/theme_name/gtk-3.0/) after the recent light-themes update. One difference I noted in the section titled notebook is given below.

Ambiance:

.notebook tab:active {
    color: @fg_color;
}

.notebook tab {
    color: shade (@bg_color, 0.48);
}

Radiance:

.notebook tab GtkLabel:active {
    color: @fg_color;
}

.notebook tab GtkLabel {
    color: shade (@bg_color, 0.48);
}

Radiance has GtkLabel but Ambiance doesn't. My question is: does anyone know the significance of GtkLabel, and why one theme has it and the other doesn't?

Solution 1:

I updated my system recently and my gtk-widgets.css are the same in this part, for Ambiance or Radiance.

Although GtkLabel is invoked to set attributes that may differ from the default pattern, so it was used to modify some defaults of tabs that may fit Ambiance, but not Radiance.

When calling gtk.Notebook you don't need to specify an attribute, it will be used a default pattern, but in case of Radiance they wanted to set a different attribute from the deafult, so they called GtkLabel that was specified prior in the css:

/*********
 * label *
 *********/
GtkLabel,
GtkLabel:insensitive {
    background-color: transparent;
}

Defining the background-color of widgets that use GtkLabel (widget that displays a small amount of text) on it, such as gtk.Notebook (a tabbed notebook container).