I can share site login later, but for now, it is also happening for me on beta.theme.co.
If I log in, go to edit the homepage, jump in to Global Options and reset the Background Color to transparent, then jump into the first Intro section on the page and reset the Background Color there to transparent as well, it shows the dark grey background:
I can’t then save the beta page to see how it looks on the front end, but disabling both of those background color declarations in the Inspector still retains the default white background.
In the editor, everything seems to have a transparent background all the way back to the dark grey styling on the html
element in the editor itself (not the preview iFrame)
I’m using Chrome on a Mac, with dark mode switched on in chrome and in cornerstone, if that helps.
Will look again tomorrow and share site if the above doesn’t help
Cheers!