image header basic fuzziness

Hello,

My current configuration WP

Version of PHP/MySQL: 7
Used theme: miteri 1.0.8
Address of the site: local intranet (wordpress 4.9.4)

Met problem: image header basic fuzziness.
Example of the theme: https://demo.themeegg.com/themes/miteri

On the official site of the theme we see that the header is already dark/fuzzy of base but can one to modify/delete it?
I looked with the inspector of mozilla (examine the element), then by looking in the code but I not how find to cancel the effect or to delete him

If somebody in the answer

Cordially

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