Hey @Lely - thanks, you are right, that’s what is causing the background color issue, sorry I should have spotted it 
However, that doesn’t explain why the site was appearing as live even though the extension was enabled. I cleared the cache both on the dashboard and in the browsers I used to test this, and I couldn’t get the site to go under construction.
In the end I had to use a third-party plugin so that the in-development site was not visible. Do you have any idea what could be causing this?