Add admin custom CSS setting (#8399)

Fix #3894

app/controllers/custom_css_controller.rb

@@ -0,0 +1,10 @@
+# frozen_string_literal: true
+
+class CustomCssController < ApplicationController
+  before_action :set_cache_headers
+
+  def show
+    skip_session!
+    render plain: Setting.custom_css || '', content_type: 'text/css'
+  end
+end