Hydro/packages/ui-default/templates/extra.css

/* {% if nav_logo_dark %} */
.nav__logo {
    background-image: url({{nav_logo_dark}}) !important
}
/* {% endif %} */

@media only screen and (-webkit-min-device-pixel-ratio: 1.5), only screen and (min-resolution: 1.5dppx), only screen and (min-resolution: 144dpi) {
    /* {% if nav_logo_dark_2x %} */
    .nav__logo, .header--mobile__domain.is--system {
        background-image: url({{nav_logo_dark_2x}}) !important
    }
    /* {% endif %} */
}