Still have this issue, it's really bad. At least introduce a simple option for providing darkmode colours. I'm assuming this not having been fixed leading to an issue on Gmail side. There should be some conversations as this doesn't bode well to both Squarespace and Gmail.
Redesigning still has issues, as you're just guessing as to what works.
Files are attached if they help anyone come up with a solution. All works as expected on Apple Mail.