OK – so Dispatch requires more then CSS to work. You will need to create 2 x new Hook Elements:
https://docs.generatepress.com/article/hooks-element-overview/
1. Hook#1 add this content:
<div class="wpsp-content-wrap">
<!-- Openeing wrapper -->
Hook: Custom Hook
Custom Hook Name: wpsp_before_header
Priority: 15
Display Rules: Entire Site
NOTE When you add the above content the editor may automatically add a closing </div>
– if it does you must delete that.
2. Hook# 2 add this content:
<!-- Close WPSP Wrapper -->
</div>
Hook: Custom Hook
Custom Hook Name: wpsp_after_content
Priority: 15
Display Rules: Entire Site