Benjamin,
I don't want to discourage posting in the forum, it is hard enough to get our developer community to be more open as it is, but for issues where you think there is a bug in our product, then the first ports of call should be the Issue Tracker and your local support center.
Is the issue reported in the Issue Tracker? A search on
https://4js.com/support/issue/?id=&product=&stat=&words=topmenu+style&Search=Search#startissue doesn't reveal any likely candidates.
Report the issue to your support center.
https://4js.com/support/support-request/. Ideally providing a minimal verifiable complete example (MVCE)
https://stackoverflow.com/help/mcve and links to documentation. In this case there are no presentation styles for TopMenu
http://4js.com/online_documentation/fjs-fgl-manual-html/#fgl-topics/c_fgl_presentation_styles_019.html so I am not sure what you are referring to. Perhaps you are trying to apply common styles
http://4js.com/online_documentation/fjs-fgl-manual-html/#fgl-topics/r_fgl_presentation_styles_common_style_attributes.html in which case read the Important note there. In any case a MVCE would make it 100% clear what you are referring to.
Also note that when deciding between support request or a forum post, support requests have our timeframe responses as per Service Level Agreement.
Reuben