Menu fixed in mobile resolution

Dear Support,
I choose to use the classic menu in fixed version and it's really good for me, but when I get the mobile resolution I really don't like the effect (here in the screenshot)

menu bad.png

Instead I would need this effect:

menu good.png

How can I get it?
Best,
Baggio

I’m afraid that modifications such as this require extensive changes to the theme’s code; I do apologize but such large changes are custom work that are beyond the scope of our technical support.