Darker style focus/hover on active buttons

Currently, hovering with mouse or setting focus on a button which is
active has same styling as on a non-active button. This results in
problems for keyboard users, who set focus on a toggle and activate it,
but cannot visually see that their action had any effect. Ditto for
mouse users hovering over a toggle and clicking it. This adds an
explicit additional style for focus/hover on active buttons.
Note that this does not address issues of browser focus remaining on a
button after a mouse click (e.g. #13971), as this will likely require
extra JavaScript to fix.

less/mixins/buttons.less

@@ -8,15 +8,31 @@
   background-color: @background;
   border-color: @border;
 
-  &:hover,
   &:focus,
-  &.focus,
+  &.focus {
+    color: @color;
+    background-color: darken(@background, 10%);
+        border-color: darken(@border, 25%);
+  }
+  &:hover {
+    color: @color;
+    background-color: darken(@background, 10%);
+        border-color: darken(@border, 12%);
+  }
   &:active,
   &.active,
   .open > .dropdown-toggle& {
     color: @color;
     background-color: darken(@background, 10%);
         border-color: darken(@border, 12%);
+
+    &:hover,
+    &:focus,
+    &.focus {
+      color: @color;
+      background-color: darken(@background, 17%);
+          border-color: darken(@border, 25%);
+    }
   }
   &:active,
   &.active,