With respect to the new test theme you sent I just discovered the CSS above doesn’t work. Between 768 and 1024px the white space under widgets stays larger than its suppose to be with margin-bottom: 0; like it was in my previous screencasts.
I removed all my CSS bar that above and it didn’t work whereas on my test site with community version it does.
As soon as I change back to the community version on my staging site the margins worked as they should also. I just needed to remove the -50px entries I used previously after adding the new CSS as shown above.
So there appears to be an issue with the new theme tweak you made to me in this respect.