Menu Style is not in theme of website

I need help with the menu style on my website. For some reason the drop-down variant menu is in the style of apple. I've tested it on Chrome and Safari and it's the same on both (see image attached). Is this something that has been affected by custom code on my site or is it a bug? I really want it to look consistent iwht the style / colours of the website but can't find out how to fix it / any discussions on any similar topics. Please help!