added additional_css to workspace index

[06chaynes]

This checks for additional_css in the workspace config and if present will build the css asset and adds it to the workspace index page allowing for theme override. I've provided an example below with resulting screenshots from testing.

oranda-workspace.json

"styles": {
  "theme": "axodark",
  "additional_css": ["./.assets/styles/oranda.css"]
}

oranda.css:

html.axo {
    /* Our theme's own colors */
    --highlight-color: rgb(89, 75, 170) !important;
    --axo-orange-color: #594BAA !important;
    --axo-pink-color: #988ECC !important;
}

Before:

After:

[shadows-withal]

This is great, thank you!