MediaWiki:Common.css

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/* CSS placed here will be applied to all skins */
/* Light mode default */
.theme-light-image {
    display: inline;
}
.theme-dark-image {
    display: none;
}

/* Dark mode via OS/browser preference */
@media (prefers-color-scheme: dark) {
    .theme-light-image {
        display: none;
    }
    .theme-dark-image {
        display: inline;
    }
}