Hi,
That sounds like the styling is applied incorrectly, which from experience tells me something (namely a plugin) is manipulating the styling or mixing it around.
I you have any caching/minifying/optimizations plugins active on the site, please disable them temporarily and check if the issue still manifests.
Otherwise, broken appearance can also be caused by an issue in the markup itself, like unclosed/unbalanced tags or incorrect tag combinations (causing the browser to try to save face by attempting to guess what starts, goes and ends where).
Without browsing the site in question I wouldn’t be able to provide any more specific insight.