build(theme): remove unnecessary dependency on @ngx-pwa/local-storage (#126)

Remove unnecessary dependency and use browser local storage to store
theme instead.

Fixes #125

package.json

@@ -37,7 +37,6 @@
     "@angular/platform-browser-dynamic": "^6.0.6",
     "@angular/router": "^6.0.6",
     "@angular/service-worker": "^6.0.6",
-    "@ngx-pwa/local-storage": "^6.0.0-rc.0",
     "@ngx-translate/core": "^10.0.2",
     "@swimlane/ngx-charts": "^8.1.0",
     "angular-archwizard": "^3.0.0",