It looks like this can be changed with `setProperty`. There&#x27;s a demo[1] that works on Firefox 31+ and Chrome 49+<p>[1] <a href="https:&#x2F;&#x2F;www.broken-links.com&#x2F;2014&#x2F;08&#x2F;28&#x2F;css-variables-updating-custom-properties-javascript&#x2F;" rel="nofollow">https:&#x2F;&#x2F;www.broken-links.com&#x2F;2014&#x2F;08&#x2F;28&#x2F;css-variables-updati...</a>