I'm starting to see this on our Forms as well; it's usually also accompanied by a form failing to auto-fill details about the user (like {_currentuser_display} and {_currentuser}) recorded in read-only auto-filled fields.

When we get reports of this, it usually goes away on it's own within a few minutes of the users calling to complain about it. It's hard to even find an Event Viewer event or log that matches these instances, so I can't figure out what is causing it.

I suspect it may be caused by the Forms server running low on resources (RAM, cache, etc.) when the request for the form is started, and thus failing to send/write one or more of the component pieces of the PHP page being rendered by the client browser. This could result in a page being sent to the user's browser that's missing some of the CSS styling sections.

Next time it happens to you, try to view (and save as text) the source of the page that's being rendered in black and white. Then compare it to a properly rendered version of the same page. See if you can find out if some part of the page is failing to be generated. If they look identical, then it could be the linked CSS files that are not being sent to the client browser; at which point I still think an over-taxed IIS/web server could be the culprit.