Hello, @Anas,
Actually, there is a CSS code added in the theme options that are causing the issue. Check the image: https://postimg.cc/62L15mvf I disable it. And then I added a custom CSS code in the theme options to set the simple drop-down menu as you need. see image: https://postimg.cc/XZgznPFc
Now, All you have to do is if you want to add another simple sub-menu on your site just add a class on the main nav menu of the sub-menu items see image: https://postimg.cc/FYbvwNrT by adding this class this menu will appear like this: https://postimg.cc/kVky838d
Regards 8Themes Team.