mathematics-physics-wiki/docs/en/stylesheets/extra.css

[data-md-color-scheme="slate"] .md-typeset blockquote {
    border-left: .2rem solid rgba(68, 138, 255, 1);
    background-color: rgba(34, 44, 63, 0.6);
    color: rgb(240, 240, 240);
}

[data-md-color-scheme="default"] .md-typeset blockquote {
    border-left: .2rem solid rgba(68, 138, 255, 1);
    background-color: rgba(34, 44, 63, 0.6);
    color: rgb(0, 0, 0);
}