Fix scroll button styling

The selector was not specific enough, and the hover styling
was missing.

gtk/theme/Adwaita/_common.scss

@@ -2293,11 +2293,15 @@ notebook {
  ************/
 
 tabs {
-  button {
+  button.flat.image-button {
     @include button(undecorated);
     border-radius: 50%;
     margin: 10px;
     padding: 0;
+    &:hover {
+      @include button(hover);
+      -gtk-icon-effect: highlight;
+    }
   }
   tab {
     min-width: 6em;

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

@@ -2573,7 +2573,7 @@ notebook > stack:not(:only-child) {
 /************
  * Tabstrip *
  ************/
-tabs button {
+tabs button.flat.image-button {
   border-color: transparent;
   background-color: transparent;
   background-image: none;
@@ -2583,6 +2583,16 @@ tabs button {
   border-radius: 50%;
   margin: 10px;
   padding: 0; }
+  tabs button.flat.image-button:hover {
+    color: #eeeeec;
+    outline-color: rgba(238, 238, 236, 0.3);
+    border-color: #1b1f20;
+    border-bottom-color: #0b0c0c;
+    background-image: linear-gradient(to bottom, #353c3e, #2e3436 60%, #2a2e30);
+    text-shadow: 0 -1px rgba(0, 0, 0, 0.77945);
+    -gtk-icon-shadow: 0 -1px rgba(0, 0, 0, 0.77945);
+    box-shadow: inset 0 1px rgba(255, 255, 255, 0.05);
+    -gtk-icon-effect: highlight; }
 tabs tab {
   min-width: 6em;
   padding: 13px;

gtk/theme/Adwaita/gtk-contained.css

@@ -2586,7 +2586,7 @@ notebook > stack:not(:only-child) {
 /************
  * Tabstrip *
  ************/
-tabs button {
+tabs button.flat.image-button {
   border-color: transparent;
   background-color: transparent;
   background-image: none;
@@ -2596,6 +2596,16 @@ tabs button {
   border-radius: 50%;
   margin: 10px;
   padding: 0; }
+  tabs button.flat.image-button:hover {
+    color: #2e3436;
+    outline-color: rgba(46, 52, 54, 0.3);
+    border-color: #b6b6b3;
+    border-bottom-color: #91918c;
+    background-image: linear-gradient(to bottom, #f7f7f7, #e8e8e7 60%, #dededd);
+    text-shadow: 0 1px rgba(255, 255, 255, 0.76923);
+    -gtk-icon-shadow: 0 1px rgba(255, 255, 255, 0.76923);
+    box-shadow: inset 0 1px white;
+    -gtk-icon-effect: highlight; }
 tabs tab {
   min-width: 6em;
   padding: 13px;