We have a 7.02 productive system and a 7.40 system that we'll be rolling out shortly. We've been working in the 7.40 system for some time and just recently started seeing this issue.
In our 7.02 system our layout previews render as follows:
In the 7.40 system they just started recently rendering like below:
We haven't done any updates to the system recently, but is there a setting somewhere that needs to be changed to get a theme or something to render the preview correctly?