Support brand packages that don't include core paragon themes

getParagonThemeCss (as well as a few other spots) assume installed brand packages include core themes and will fail when presented with a brand package that is aliased without them.

Ideally a brand package with a few overrides could have the following theme-urls.json

{
  "themeUrls": {
    "defaults": {
      "light": "light"
    },
    "variants": {
      "light": {
        "paths": {
          "default": "./light.css",
          "minified": "./light.min.css"
        }
      }
    }
  }
}

and still work when installed using an npm alias.

Start by reading getParagonThemeCss and the other spots that assume brand packages contain core themes. Reproduce the issue with the provided theme-urls.json structure and an npm-alias installation. Done means a brand package containing only overrides loads its light theme without failing.