Made dimmed text more visible in Gnome Boxes with Ambiance
In Gnome Boxes (specifically in New > Enter URL), dimmed text is used into a GtkBox whose background is way darker than the @bg_color respect which the dim-label is defined in gtk-widget.css. This makes the dim-label very hard to see.
To fix it, I added a new gnome-boxes.css with a darker definition for dim-label that are specifically children of such GtkBox class (boxes-menu-row). (LP: #1700100)
Approved by: Marco Trevisan (Treviño)