GtkSpinner widget remains persistently visible by default

[TheBoctor]

In this part of the base.css stylesheet for the GTK3 (and 4) style, the GTK spinner widget is set to be visible in its default state. I am not sure if this is what you intend, but in the Thunar file manager, this causes an inactive spinner icon to remain persistently visible in the top-right corner of the application. The default behavior of this widget in other themes is to remain hidden until active/checked.

The visual quirk is resolved if this block of CSS is replaced by the following:

spinner {
        color: @theme_fg;
        opacity: 0;
}

Thanks so much for the time and care you've given to keeping these themes alive.