html[data-theme=dark]

#promo {
    content:url('/img/demo_black.png')
}

html[data-theme=light] #promo{content:url('/img/demo_white.png')}  code[class*=language-],pre[class*=language-]{font-family:Consolas,Monaco,andale mono,ubuntu mono,monospace;text-align:left;white-space:pre;word-spacing:normal;word-break:normal;word-wrap:normal;line-height:1.5;-moz-tab-size:4;-o-tab-size:4;tab-size:4;-webkit-hyphens:none;-moz-hyphens:none;-ms-hyphens:none;hyphens:none}  pre[class*=language-]{padding:1em;margin:.5em 0;overflow:auto}  :not(pre)>code[class*=language-]{padding:.1em;border-radius:.3em;white-space:normal}  .token.comment,.token.prolog,.token.doctype,.token.cdata{color:#708090}  .token.punctuation{color:#999}  .namespace{opacity:.7}  .token.property,.token.tag,.token.boolean,.token.constant,.token.symbol,.token.deleted{color:#905}  .token.selector,.token.number,.token.attr-name,.token.string,.token.char,.token.builtin,.token.inserted{color:#690}  .token.operator,.token.entity,.token.url,.language-css .token.string,.style .token.string{color:#9a6e3a}  .token.atrule,.token.attr-value,.token.keyword{color:#07a}  .token.function,.token.class-name{color:#dd4a68}  .token.regex,.token.important,.token.variable{color:#e90}  .token.important,.token.bold{font-weight:700}  .token.italic{font-style:italic}  .token.entity{cursor:help}