Background is already set on the outer <figure>-element:

figure[typeof~='mw:File/Thumb'],
figure[typeof~='mw:File/Frame'] {
 border:var(--border-subtle,1px solid #c8ccd1);
 border-bottom:0;
 background-color:var(--background-color-interactive-subtle,#f8f9fa);
}

I don't see any reason why the same background is set again on the <img>:

figure[typeof~="mw:File/Thumb"] > :not(figcaption) .mw-file-element, 
figure[typeof~="mw:File/Frame"] > :not(figcaption) .mw-file-element {
	[...]
	background: var(--background-color-interactive-subtle,#f8f9fa);
}

Removing that line would solve this issue, as .skin-invert-image won't invert anything that is defined outside the <img>.

(Code according to Firefox DevTools; don't know where to find it in the repo.)

It works fine with Minerva, this problem only concerns Vector 2022

Do you have an estimate of when this will be resolved? Given the imminent night mode, onwiki documentation needs to be drawn up for contributors. It would be useful to have the class up and running to explain how it works.