gray gradient between menu and header image

hello everyone is a beginner
I would like to remove the gray nuance between the menu and the header image.
This nuance created by the menu makes the image of the header and also the other images in evidence of each darker page at the bottom.
Then scrolling down the image you see better.
I want to remove this nuance because when I look at my site especially from a smartphone, the image of the header (white in my case) is very dark!

I hope I have explained myself because I am Italian and I am using a translator.

Thanks a lot for the collaboration

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