diff --git a/gtk/resources/theme/Adwaita/_common.scss b/gtk/resources/theme/Adwaita/_common.scss
index 0c50fa31b7..8350dc17a9 100644
--- a/gtk/resources/theme/Adwaita/_common.scss
+++ b/gtk/resources/theme/Adwaita/_common.scss
@@ -1613,6 +1613,7 @@ GtkLevelBar.vertical {
  **********/
 .frame {
   border: 1px solid $borders_color;
+  &.flat { border-style: none; }
   &:backdrop { border-color: $backdrop_borders_color; }
   padding: 0;
   &.action-bar {
diff --git a/gtk/resources/theme/Adwaita/gtk-contained-dark.css b/gtk/resources/theme/Adwaita/gtk-contained-dark.css
index 3b6ccc6047..62ccb6f5d5 100644
--- a/gtk/resources/theme/Adwaita/gtk-contained-dark.css
+++ b/gtk/resources/theme/Adwaita/gtk-contained-dark.css
@@ -2183,6 +2183,8 @@ GtkLevelBar.vertical {
 .frame {
   border: 1px solid #1c1f1f;
   padding: 0; }
+  .frame.flat {
+    border-style: none; }
   .frame:backdrop {
     border-color: #1e2222; }
   .frame.action-bar {
diff --git a/gtk/resources/theme/Adwaita/gtk-contained.css b/gtk/resources/theme/Adwaita/gtk-contained.css
index 6123cefed2..76341b22c2 100644
--- a/gtk/resources/theme/Adwaita/gtk-contained.css
+++ b/gtk/resources/theme/Adwaita/gtk-contained.css
@@ -2181,6 +2181,8 @@ GtkLevelBar.vertical {
 .frame {
   border: 1px solid #a1a1a1;
   padding: 0; }
+  .frame.flat {
+    border-style: none; }
   .frame:backdrop {
     border-color: #a8a8a8; }
   .frame.action-bar {