Hi kaktuseek,
Thank you for buying our theme. We hope it will meet all your expectations! 🙂
Some colors used in this dropdown are variables. To change this, I suggest you to set different colors to the submenu in Customize > Appearance > Navigation > Item. Please find the attached screenshot.
Could you please make sure to select a dark color for the text and a light color as background? These changes should solve your issue. In case you’re still facing the same issue, please make sure to clear all activated caching system as well as your browser cache before investigating any further. If you want to read more on what browser cache is and how to clear it, here is an article that might help you: How to Clear Internet Cache in Every Major Browser.
Thank you.