Include custom stylesheet

Please add to the SPA template something like

```

```

at the end of the header. This would allow me to server `/local/styles.css` from NGinX and do a little bit of customisation, i.e. change the default colour scheme for each domain etc.

Locate the SPA template and inspect where the header is assembled. Add the requested stylesheet link at the end of the header, then verify that the generated SPA references /local/styles.css and that the existing template output remains intact.

Written by the indexing model from the issue text.