Style horizontal separators only

[Serrano Pereira <serrano pereira gmail com>]

Hello,

I'm trying to style horizontal separators in a custom GTK3 application
using the following stylesheet:

.separator.horizontal {
    border-width: 1px;
    border-style: dashed;
}

But it has no effect. If I leave out the 'horizontal' class it works
fine, but then it also styles the vertical separators, which I don't want.

How can I style just the horizontal separators? According to the
documentation GtkSeparator implements GtkOrientable, which gets the
'horizontal' CSS class.

Regards,
Serrano