Add btn-default class for Service Desk toggle in settings

app/views/projects/_service_desk_settings.html.haml

@@ -2,7 +2,7 @@
 %section.settings.js-service-desk-setting-wrapper.no-animate#js-service-desk{ class: ('expanded' if expanded), data: { qa_selector: 'service_desk_settings_content' } }
   .settings-header
     %h4.settings-title.js-settings-toggle.js-settings-toggle-trigger-only= _('Service Desk')
-    %button.btn.gl-button.js-settings-toggle
+    %button.btn.gl-button.btn-default.js-settings-toggle
       = expanded ? _('Collapse') : _('Expand')
     - link_start = "<a href='#{help_page_path('user/project/service_desk')}' target='_blank' rel='noopener noreferrer'>".html_safe
     %p= _('Enable and disable Service Desk. Some additional configuration might be required. %{link_start}Learn more%{link_end}.').html_safe % { link_start: link_start, link_end: '</a>'.html_safe }