one-pager website: Logo in header resizes

Hi,
I guess this is a feature of header in one-pager page template but is giving me headache.
I have used a logo in the header (Appearance – Customizer). No matter how small image I use (have gone to 32 pixels as well), but every time during page scroll, the logo shrinks in size. It goes back to the original size once you reach the top of page.

I would like it to remain the same size at it is displayed when you first open the website (before the scroll).

I am hoping it is just a tick mark somewhere and I would not have to go into the code. Never done html/php/java coding. So if the solution involves coding, please give as detailed instructions as you possibly can (including the filename).
Thanks.