Dictionary styling (background color)
Here's the content of DefaultStyle.css file which I put in Contents subdirectory for each dictionary I have at ~/Library/Dictionaries and at /System/Library/Assets/com_apple_MobileAsset_DictionaryServices_dictionaryOSX/:

@charset "UTF-8";
@namespace d url(http://www.apple.com/DTDs/DictionaryService-1.0.rng);

@media (prefers-dark-interface)
{

    html {
        -apple-color-filter: apple-invert-lightness();
    }
    a {
        -apple-color-filter: none;
        color: -webkit-link;
    }
    img {
        filter: invert(100%);
    }
}


It fixed the problem of background color for me, which appeared in Apple Dictionary somewhat after I migrated to Mojave.

I think you can edit background-color in this CSS as you want to achieve the result.