Use storage.local instead of window.localStorage for styles storage

[yfdyh000]

I noticed the styles was lost if I removed the webappsstore.sqlite file for some purpose. e.g. it is too large and I need to rebuild it.

https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/storage/local
Although this API is similar to Window.localStorage it is recommended that you don't use Window.localStorage in extension code. Firefox will clear data stored by extensions using the localStorage API in various scenarios where users clear their browsing history and data for privacy reasons, while data saved using the storage.local API will be correctly persisted in these scenarios.