JSFiddle - React, Tailwind, and code Playground

HTML

<p class="element" style="margin: 10px">
  Element
</p>

CSS

.element {
  background: red;
  margin: 20px;
}

JavaScript

var elm = document.getElementsByClassName("element");

function getStyle(className_) {

  var styleSheets = window.document.styleSheets;
  var styleSheetsLength = styleSheets.length;
  for(var i = 0; i < styleSheetsLength; i++){
    var classes = styleSheets[i].rules || styleSheets[i].cssRules;
    if (!classes)
      continue;
    var classesLength = classes.length;
    for (var x = 0; x < classesLength; x++) {
      if (classes[x].selectorText == className_) {
        var ret;
        if(classes[x].cssText){
          ret = classes[x].cssText;
        } else {
          ret = classes[x].style.cssText;
        }
        if(ret.indexOf(classes[x].selectorText) == -1){
          ret = classes[x].selectorText + "{" + ret + "}";
        }
        return ret;
      }
    }
  }

}
//from: https://stackoverflow.com/a/27527462/5814976

var cssElm = getStyle('.element'),
		cssVal = cssElm.split('margin:')[1].split(';')[0],
		inlineVal = elm[0].style.margin,
    totalVal = parseInt(cssVal.replace(/[^0-9.]/g, "")) + parseInt(inlineVal.replace(/[^0-9.]/g, ""));
    
    console.log(totalVal)

    elm[0].style.margin = totalVal + 'px';