Adwaita: Update flowbox rubberband styling

Adapt to the changes in the previous commit.

gtk/theme/Adwaita/_common.scss

@@ -126,6 +126,10 @@ rubberband {
   background-color: transparentize(darken($selected_bg_color, 10%), 0.8);
 }
 
+flowbox {
+  & rubberband { @extend rubberband; }
+}
+
 label {
   &.separator {
     color: $fg_color;

gtk/theme/Adwaita/gtk-contained-dark.css

@@ -79,6 +79,7 @@ textview text,
 
 .rubberband,
 rubberband,
+flowbox rubberband,
 treeview.view.rubberband,
 .content-view rubberband {
   border: 1px solid #184472;

gtk/theme/Adwaita/gtk-contained.css

@@ -79,6 +79,7 @@ textview text,
 
 .rubberband,
 rubberband,
+flowbox rubberband,
 treeview.view.rubberband,
 .content-view rubberband {
   border: 1px solid #2a76c6;