feat: Dark mode implemented

[psankhe28]

What

I added a dark mode to the user settings file, resolved the footer issue from the previous pull request, eliminated the "Follow System Settings" option, and aligned the dark mode with the system default value.

Screenshot

Dark mode:

Light mode:

System mode:

Part of https://github.com/openfoodfacts/open-prices-frontend/issues/274

[psankhe28]

I am currently in the process of modifying and reviewing various components, as well as checking different pages. I will push the changes as soon as possible.

[raphodn]

Hi @psankhe28 ! We should rather use the built-in vuetify dark theme mode : https://vuetifyjs.com/en/features/theme/#setup

[Valimp]

Hi @psankhe28 😄 Are you still working on this feature ?

[raphodn]

Closing in favor of #921 (using the "official" vuetify way)