target header background image only

Hi,

I am trying to apply a transition to my header background-image, but so far I have been able to target the site-header, which is not what I want.

I am using this

.site-header {
  -webkit-animation: brightness 4s ease-in;
}
@-webkit-keyframes brightness {
  0%,100%  { -webkit-filter: brightness(0%); }
  100% { -webkit-filter: brightness(100%); }
 }

But as you can see it applies to every thing including the navigation, and as I said earlier I want this only and exclusively to be applied to the header back-ground-image.

If it was an ordinary content I could apply a class and so on, but in this case I have no idea how to target it.

Thanks in advance for any help

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