Support brand packages that don't include core paragon themes

[brian-smith-tcril]

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.