Unescaped '#' in data URI bodies

Unescaped `#` in data URI bodies has been deprecated in Chrome since version 71.

This caused several CSS background images on this site to fail to render as they used data URIs of SVG images with attributes such asfill='#00ff00'. The fix is to replace the `#` with `%23`.

If you have experienced any SVG data URI background images disappearing lately this might be something to look out for!