From https://once.com/writebook

app/controllers/accounts/custom_styles_controller.rb

@@ -0,0 +1,20 @@
+class Accounts::CustomStylesController < ApplicationController
+  before_action :ensure_can_administer, :set_account
+
+  def edit
+  end
+
+  def update
+    @account.update!(account_params)
+    redirect_to edit_account_custom_styles_url
+  end
+
+  private
+    def set_account
+      @account = Current.account
+    end
+
+    def account_params
+      params.require(:account).permit(:custom_styles)
+    end
+end