Channel switcher UI improvements (#6564)

* Channel switcher UI improvements

* Reverting mattermost theme

* Fixing spacing above the button

* Updating switcher button

* PLT-6691 - Fixing tablet header

webapp/sass/components/_modal.scss

@@ -79,6 +79,7 @@
         .channel-switch-modal {
             .modal-header {
                 background: transparent;
+                border: none;
                 min-height: 0;
                 padding: 0;