Merge branch 'dz-fix-sidebar-overlap' into 'master'

Fix left sidebar overlap when scroll on long menu

Fixes #17566. Caused by https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4139. Smaller fix compared to https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4164

cc @iamphill @jschatz1 
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

See merge request !4165

app/assets/stylesheets/framework/gitlab-theme.scss

@@ -9,6 +9,8 @@
 @mixin gitlab-theme($color-light, $color, $color-darker, $color-dark) {
   .page-with-sidebar {
     .header-logo {
+      background: $color-darker;
+
       a {
         color: $color-light;