I tried to make dotted border separators for tabs:

.notebook tab,
.notebook tab:backdrop {
    /* ... */
    border-right: 1px dotted @borders;
}

Which seems to completely throw off all drawing (screenshot attached) with messages like these:

(gedit:7356): Gtk-WARNING **: drawing failure for widget `GtkPaned': invalid value for a dash setting

(gedit:7356): Gtk-WARNING **: drawing failure for widget `GtkBox': invalid value for a dash setting

(gedit:7356): Gtk-WARNING **: drawing failure for widget `GeditWindow': invalid value for a dash setting

(gedit:7356): Gtk-WARNING **: drawing failure for widget `GeditNotebook': invalid value for a dash setting


using border-style property does the same.