updated buttons in the app/views/shared/milestones directory

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

 - milestone_url = @milestone.project_milestone? ? project_milestone_path(@project, @milestone) : group_milestone_path(@group, @milestone)
 
-%button.js-delete-milestone-button.btn.btn-grouped.btn-danger{ data: { milestone_id: @milestone.id,
+%button.js-delete-milestone-button.btn.gl-button.btn-grouped.btn-danger{ data: { milestone_id: @milestone.id,
   milestone_title: markdown_field(@milestone, :title),
   milestone_url: milestone_url,
   milestone_issue_count: @milestone.issues.count,

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

@@ -11,10 +11,10 @@
 
   .milestone-buttons
     - if can?(current_user, :admin_milestone, @group || @project)
-      = link_to _('Edit'), edit_milestone_path(milestone), class: 'btn btn-grouped'
+      = link_to _('Edit'), edit_milestone_path(milestone), class: 'btn gl-button btn-grouped'
 
       - if milestone.project_milestone? && milestone.project.group
-        %button.js-promote-project-milestone-button.btn.btn-grouped{ data: { toggle: 'modal',
+        %button.js-promote-project-milestone-button.btn.gl-button.btn-grouped{ data: { toggle: 'modal',
           target: '#promote-milestone-modal',
           milestone_title: milestone.title,
           group_name: milestone.project.group.name,
@@ -26,11 +26,11 @@
         #promote-milestone-modal
 
       - if milestone.active?
-        = link_to _('Close milestone'), update_milestone_path(milestone, { state_event: :close }), method: :put, class: 'btn btn-grouped btn-close'
+        = link_to _('Close milestone'), update_milestone_path(milestone, { state_event: :close }), method: :put, class: 'btn gl-button btn-grouped btn-close'
       - else
-        = link_to _('Reopen milestone'), update_milestone_path(milestone, { state_event: :activate }), method: :put, class: 'btn btn-grouped btn-reopen'
+        = link_to _('Reopen milestone'), update_milestone_path(milestone, { state_event: :activate }), method: :put, class: 'btn gl-button btn-grouped btn-reopen'
 
       = render 'shared/milestones/delete_button'
 
-    %button.btn.btn-default.btn-grouped.float-right.d-block.d-sm-none.js-sidebar-toggle{ type: 'button' }
+    %button.btn.gl-button.btn-default.btn-grouped.float-right.d-block.d-sm-none.js-sidebar-toggle{ type: 'button' }
       = sprite_icon('chevron-double-lg-left')

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

@@ -8,7 +8,7 @@
         = markdown_field(label, :description)
 
       .float-right.d-none.d-lg-block.d-xl-block
-        = link_to milestones_issues_path(options.merge(state: 'opened')), class: 'btn btn-transparent btn-action' do
+        = link_to milestones_issues_path(options.merge(state: 'opened')), class: 'btn gl-button btn-default-tertiary btn-action' do
           - pluralize milestone_issues_by_label_count(@milestone, label, state: :opened), _('open issue')
-        = link_to milestones_issues_path(options.merge(state: 'closed')), class: 'btn btn-transparent btn-action' do
+        = link_to milestones_issues_path(options.merge(state: 'closed')), class: 'btn gl-button btn-default-tertiary btn-action' do
           - pluralize milestone_issues_by_label_count(@milestone, label, state: :closed), _('closed issue')

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

@@ -46,7 +46,7 @@
       .milestone-actions.d-flex.justify-content-sm-start.justify-content-md-end
         - if @project # if in milestones list on project level
           - if can_admin_group_milestones?
-            %button.js-promote-project-milestone-button.btn.btn-blank.btn-sm.btn-grouped.has-tooltip{ title: s_('Milestones|Promote to Group Milestone'),
+            %button.js-promote-project-milestone-button.btn.gl-button.btn-default-tertiary.btn-sm.btn-grouped.has-tooltip{ title: s_('Milestones|Promote to Group Milestone'),
               disabled: true,
               type: 'button',
               data: { url: promote_project_milestone_path(milestone.project, milestone),
@@ -59,6 +59,6 @@
 
         - if can?(current_user, :admin_milestone, milestone)
           - if milestone.closed?
-            = link_to s_('Milestones|Reopen Milestone'), milestone_path(milestone, milestone: { state_event: :activate }), method: :put, class: "btn btn-sm btn-grouped btn-reopen"
+            = link_to s_('Milestones|Reopen Milestone'), milestone_path(milestone, milestone: { state_event: :activate }), method: :put, class: "btn gl-button btn-sm btn-grouped btn-reopen"
           - else
-            = link_to s_('Milestones|Close Milestone'), milestone_path(milestone, milestone: { state_event: :close }), method: :put, class: "btn btn-sm btn-grouped btn-close"
+            = link_to s_('Milestones|Close Milestone'), milestone_path(milestone, milestone: { state_event: :close }), method: :put, class: "btn gl-button btn-warning-secondary btn-sm btn-grouped btn-close"