Re: CSS mystery…
Yes: child-theme of bp-default.
child-theme/header calls child-theme/style.css and child-theme/custom.css
child-theme/style.css imports bp-default/style.css
So if I remove the background property from, say div#container in child-theme/style.css, it gets that background from bp-default. Then if I add the background property to child-theme/custom.css, nothing changes, even with !important added.
Like I say, I’m stumped 
