i would suspect the css is loading after the page renders, or maybe he has overridden the css in some external file? im not sure how your guy has gone about his styling, but i have never seen another xenforo do that. ive seen it on websites before, but not an xf. its really weird seeing it happen. you should definitely point that out to whomever does your styling, and point out that you cannot find another xf that does it.
its difficult for me to really troubleshoot it because i moved house and am using subpar 3g for my internet for 2-3 weeks.