Adwaita: notebook arrow styling.

gtk/theme/Adwaita/_common.scss

@@ -393,6 +393,7 @@ $_dot_color: if($variant=='light', $selected_bg_color,
   }
 }
 
+%button,
 button {
 
   $_button_transition: all 200ms $ease-out-quad;
@@ -1831,8 +1832,36 @@ notebook {
       }
     }
 
-    arrow:disabled {
-      color: $insensitive_fg_color;
+    > tabs > arrow {
+      @extend %button;
+      @extend button.flat;
+
+      min-height: 16px;
+      min-width: 16px;
+      margin-left: -5px;
+      margin-right: -5px;
+      padding-left: 4px;
+      padding-right: 4px;
+      border-top-style: none;
+      border-radius: 0;
+
+      &:hover:not(:active):not(:backdrop) {
+        background-image: none;
+        background-color: transparentize(white, 0.7);
+        border-left-color: transparent;
+        border-right-color: transparent;
+        box-shadow: none;
+      }
+
+      &:disabled {
+        background-image: none;
+        background-color: transparent;
+        border-color: transparent;
+      }
+
+      &.down { -gtk-icon-source: -gtk-icontheme('pan-start-symbolic'); }
+
+      &.up { -gtk-icon-source: -gtk-icontheme('pan-end-symbolic'); }
     }
 
     tab {