feat(core): add default values to the theme even if the user already have some in the library

[gabrieleAngius]

Is your feature request related to a problem? Please describe.
This issue is related to the following comment on #17

Basically as suggested by @lucacacciarru we should consider if we want to add some default values anyway

Describe the solution you'd like
Basically IMO it's easier to add the defaults just if the library have not valid values for two reasons:

• in a already-existing project, most likely the user want to see his values, so will be forced to delete the defaults if don't need them
• if we add the defaults anyway, resetting the widget / deleting it and restarting will case duplications on the library (so it'll require a pretty heavy check to understand if the defaults are already in the library)

Describe alternatives you've considered
Probably the cleanest option could be to ask the user if he wants the default values or not, through some config (or maybe even a "generate defaults" button)

Let's discuss it here to find the best solution. Comment from @mauroerta is very welcome ❤️