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-lock.json

@@ -685,11 +685,6 @@
         "webpack-sources": "1.1.0"
       }
     },
-    "@ngx-pwa/local-storage": {
-      "version": "6.0.0-rc.0",
-      "resolved": "https://registry.npmjs.org/@ngx-pwa/local-storage/-/local-storage-6.0.0-rc.0.tgz",
-      "integrity": "sha512-ks+5nSjSbErv8u22jsu4zKUDcQuXTum9k5qVptq2+mytzxXfNZLMNz3NT91Ww72axgsoDKBNEYNaLuY4ST5pLg=="
-    },
     "@ngx-rocket/ascii-logo": {
       "version": "1.1.0",
       "resolved": "https://registry.npmjs.org/@ngx-rocket/ascii-logo/-/ascii-logo-1.1.0.tgz",