Hello and thank you for contacting the Toolset support.
Please check the following articles, they explain how to add custom CSS to WordPress:
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Note, that Toolset views, content template, and archive templates, all have a section to add custom CSS on them. But I don't think this can help you fix the top menu for the whole site. Maybe one of the solutions above can help.
I hope this helps. Let me know if you still need assistance with this.