This has to be relative, because we don't know if riot is going to be mounted
at the top-level of the domain or not (it's not, on riot.im).
Links are relative to the final location of the CSS, which is under
bundles/<hash>, so need ../..
SVGs are all broken, and some of the more exotic colours haven't been updated. There's been no attempt to use SASS to remove duplication from the CSS yet.
no attempt to switch it at runtime yet.
