Add support for var() CSS function

[weltkante]

I don't know exactly what it does but I have some svg files with things like var(--color-text, #000) that fail to load. I can work around it by doing a regex-replacement throughout the svg, so its not blocking me, just reporting it in case you think this should be supported. If its out of scope feel free to close.

Example (which is just a black bar):

<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 1000 1000">
  <path d="M60 500h880" style="fill:none;stroke:var(--color-text, #000);stroke-linecap:square;stroke-miterlimit:2;stroke-width:68px" />
</svg>

[wieslawsoltes]

The CSS var is not supported https://developer.mozilla.org/en-US/docs/Web/CSS/var