How to make modal login box fixed unmoved ?

Dear Flynax Developers,

How do I make the login box in my flynax site fixed just the way it does in flynax home page. Please refer to picture 1 and 2

Attachment 1026
In picture 1 above is when i first click on the "Customer Login" on the top left of flynax home page, the box appear in the middle of the screen.

Attachment 1027
In Picture 2 above, I can scroll the page down and the box maintain in the same position, fixed. That is why the box is now appear at top because i scrolled the page down

Attachment 1028
In picture 3 above, when i clicked login the box appear at the middle similar to flynax home page.

Attachment 1029
However, in picture 4 above, as I scroll down, the box move as i scroll.

How do I make the login box fixed at the top, similar to flynax hompage login, and not moving around when i scroll?
I have tried around and I think the code is in style.css, in this part:

Code:

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Only registered members can view the code.

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But I don't know how to make the change.
Please advice

Thank you and have a good day