responsive menu

Hello, I hope is the good place to ask this question…

I plan to buy the theme U design theme.
I like the menu in this exemple :
At the start, the menu appears at the top in transparency but not pasted at the top and at the right of the screen.
Then when you go down the page, the menu turns into a horizontal bar that occupies the entire screen.

The problem is that in the “smartphone size” mode, the menu does not remain frozen at the top of the page when moving within the page (unlike the “PC screen size” mode). Is it possible to freeze at the top of the screen the menu in “smartphone size” mode while keeping the current operation of the menu bar in “PC screen size” mode (ie upper right and Transparent when one is at the top of the website)?

In summary: whether on a smartphone or on a PC I would like the menu always visible.
Thank you


Better to ask the question here