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https://stackoverflow.com/questions/4724606/how-to-use-javascript-to-check-and-load-css-if-not-loaded

// check if to.css is loaded
if (!$("link[href='/path/to.css']").length)
    $('<link href="/path/to.css" rel="stylesheet">').appendTo("head");



if ( !jQuery("link[href='/wp-content/themes/nextdoor/night.css']").length ) {
    // at 6:59PM day style sheet is active, when it gets to this block it needs to be disabled first
    jQuery('link[href="/wp-content/themes/nextdoor/day.css"]').prop('disabled', true);
    // then load night Time stylesheet
    jQuery("head link[rel='stylesheet']").last().after("<link id='night_style' rel='stylesheet' href='/wp-content/themes/nextdoor/night.css' type='text/css'>");
}