UX: adjust various button classes (#24121)

app/assets/javascripts/discourse/app/components/emoji-picker.hbs

@@ -11,7 +11,7 @@
           {{on "click" (fn this.onCategorySelection "recent")}}
           data-section="recent"
           type="button"
-          class="btn btn-default category-button emoji"
+          class="btn btn-flat category-button emoji"
         >
           {{replace-emoji ":star:"}}
         </button>

app/assets/javascripts/discourse/app/templates/preferences/interface.hbs

@@ -247,7 +247,7 @@
     <DButton
       @action={{action "resetSeenUserTips"}}
       data-setting-name="user-reset-seen-user-tips"
-      class="pref-reset-seen-user-tips"
+      class="btn-default pref-reset-seen-user-tips"
     >{{i18n "user.reset_seen_user_tips"}}</DButton>
   {{/if}}
 </fieldset>

app/assets/javascripts/discourse/app/templates/user-invited-show.hbs

@@ -21,7 +21,7 @@
           @icon="plus"
           @action={{this.createInvite}}
           @label="user.invited.create"
-          class="btn-default"
+          class="btn-flat"
         />
         {{#if this.canBulkInvite}}
           {{#if this.siteSettings.allow_bulk_invite}}

app/assets/javascripts/select-kit/addon/components/notifications-filter/notifications-filter-header.js

@@ -3,7 +3,7 @@ import discourseComputed from "discourse-common/utils/decorators";
 import DropdownSelectBoxHeaderComponent from "select-kit/components/dropdown-select-box/dropdown-select-box-header";
 
 export default DropdownSelectBoxHeaderComponent.extend({
-  classNames: ["notifications-filter-header"],
+  classNames: ["notifications-filter-header", "btn-flat"],
   label: fmt("value", "user.user_notifications.filters.%@"),
 
   @discourseComputed("selectKit.isExpanded")