Hello Dan,
It seems your Search Banner plugin outlet is on the default above-main-container? I think you need to change this to place the Search Banner out from the #main-outlet 
However the theme has a chat custom style when enabled on the #main-outlet which I think only need to be active on chat pages.
This is adding the background with !important to the #main-outlet which override the theme background transparency on #main-outlet etc…
I think this is would be better with restrict this to .has-full-page-chat so only appears on chat pages?
With above-main-container setting
With below-site-header setting
