Added download-button class and applied button margin

app/assets/stylesheets/pages/tree.scss

@@ -169,4 +169,8 @@
   margin-top: 11px;
   position: relative;
   z-index: 2;
+
+  .download-button {
+    margin-left: $btn-side-margin;
+  }
 }

app/views/projects/buttons/_download.html.haml

 - if !project.empty_repo? && can?(current_user, :download_code, project)
-  %span{class: 'hidden-xs hidden-sm'}
+  %span{class: 'hidden-xs hidden-sm download-button'}
     .dropdown.inline
       %button.btn{ 'data-toggle' => 'dropdown' }
         = icon('download')