Updated styling of promote project milestone button in the milestones list

app/views/shared/milestones/_milestone.html.haml

@@ -18,10 +18,10 @@
       .float-right.light #{milestone.percent_complete(current_user)}% complete
     .col-sm-2
       - if @project
-        .col-sm-6.milestone-actions
+        .milestone-actions.d-flex.justify-content-sm-start.justify-content-md-end
           - if can?(current_user, :admin_milestone, milestone.project) and milestone.active?
             - if @project.group
-              %button.js-promote-project-milestone-button.btn.btn-sm.btn-grouped.has-tooltip{ title: _('Promote to Group Milestone'),
+              %button.js-promote-project-milestone-button.btn.btn-transparent.btn-sm.btn-grouped.has-tooltip{ title: _('Promote to Group Milestone'),
                 disabled: true,
                 type: 'button',
                 data: { url: promote_project_milestone_path(milestone.project, milestone),
@@ -30,7 +30,7 @@
                         target: '#promote-milestone-modal',
                         container: 'body',
                         toggle: 'modal' } }
-                = _('Promote')
+                = sprite_icon('level-up', size: 12)
 
             = link_to 'Close Milestone', project_milestone_path(@project, milestone, milestone: {state_event: :close }), method: :put, remote: true, class: "btn btn-sm btn-close btn-grouped"