Merge branch 'dz-small-ui-improvements' into 'master'

Small ui improvements

* make file tree row 2px smaller for more compact view
* make sub navigation  background a bit darker for better contrast with main navigation

cc @hazelyang @alfredo1 

See merge request !5315

app/assets/stylesheets/framework/variables.scss

@@ -16,7 +16,7 @@ $border-color:          #e5e5e5;
 $focus-border-color:    #3aabf0;
 $table-border-color:    #f0f0f0;
 $background-color:      #fafafa;
-$dark-background-color: #f7f7f7;
+$dark-background-color: #f5f5f5;
 $table-text-gray:       #8f8f8f;
 
 /*

app/assets/stylesheets/pages/tree.scss

@@ -19,7 +19,7 @@
       border-top: 1px solid $table-border-gray;
 
       td, th {
-        line-height: 23px;
+        line-height: 21px;
       }
 
       &:hover {