JSFiddle - React, Tailwind, and code Playground

CSS

.myClass {
    color: red;
    border: 1px solid yellow;
}

JavaScript

function getExternalCSS(className) {
    var cssText = false;
    $.each(document.styleSheets, function(key, value) {
        $.each(value.cssRules, function(k, v) {
            if(v.selectorText == className) {
                cssText = v.cssText;
                return false;
            }
        });
        if(cssText !== false) return false;
    });
    return cssText;
}

console.log(getExternalCSS('.myClass'));