Max-width: 900px is not being applied when I set the content area width to 900px. If I set it to 895, the css gets applied correctly.
I believe this may be a bug.
I created two templates with these settings, and compared the results, but there was no anomalies. Both templates were exactly the same, except the expected differences in the max-width. Can you please open a ticket and submit your template for our development team to review? Thanks!