Add some hover style

Just so we can see it is there.

gtk/theme/Adwaita/_common.scss

@@ -2308,10 +2308,15 @@ tabs {
     padding: 13px; // keeping width when paging buttons show up
     border-right: 1px solid transparentize($borders_color,0.5);
     &:first-child { border-left: 1px solid transparentize($borders_color,0.5); }
-    &:checked { //FIXME not really working
+    &:checked,
+    &:checked:hover { //FIXME not really working
       border-bottom: 2px solid $selected_bg_color;
       padding-bottom: 11px;
     }
+    &:hover {
+      border-bottom: 2px solid $borders_color;
+      padding-bottom: 11px;
+    }
   }
 }
 

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

@@ -2599,9 +2599,12 @@ tabs tab {
   border-right: 1px solid rgba(27, 31, 32, 0.5); }
   tabs tab:first-child {
     border-left: 1px solid rgba(27, 31, 32, 0.5); }
-  tabs tab:checked {
+  tabs tab:checked, tabs tab:checked:hover {
     border-bottom: 2px solid #215d9c;
     padding-bottom: 11px; }
+  tabs tab:hover {
+    border-bottom: 2px solid #1b1f20;
+    padding-bottom: 11px; }
 
 /**************
  * Scrollbars *

gtk/theme/Adwaita/gtk-contained.css

@@ -2612,9 +2612,12 @@ tabs tab {
   border-right: 1px solid rgba(182, 182, 179, 0.5); }
   tabs tab:first-child {
     border-left: 1px solid rgba(182, 182, 179, 0.5); }
-  tabs tab:checked {
+  tabs tab:checked, tabs tab:checked:hover {
     border-bottom: 2px solid #4a90d9;
     padding-bottom: 11px; }
+  tabs tab:hover {
+    border-bottom: 2px solid #b6b6b3;
+    padding-bottom: 11px; }
 
 /**************
  * Scrollbars *