Css Variable hack

gistfile1.js

function toHex(rgb){
  var a = /rgb\((\d+),\s*(\d+),\s*(\d+)\)/.exec(rgb).reverse();
  a.pop();
  return '#'+a.map(a => parseInt(a, 10).toString(16)).join('');
}

var variables = {
  '#747474': '#FF0000',
  '#f8b912': '#00FFBB'
};

Array.from(document.styleSheets).forEach(sheet=>
Array.from(sheet.cssRules)
.filter(r => r.style)
.map(r => Array.from(r.style)
     .filter(s => r.style[s] && /rgb\(\d+,\s*\d+,\s*\d+\)/.test(r.style[s]))
     .map(s => ({key: s, value: toHex(r.style[s]), original:r.style[s]}))
     .filter(({value}) => value in variables)
     .map(({key, value, original}) => {r.style[key] = original.replace(/rgb\(\d+,\s*\d+,\s*\d+\)/, variables[value]); return key}))
.filter(a => a.length))