Add Dark mode/dark theme to Application

[data-sync-user]

Users would like to have the ability to change the theme of Mozilla VPN to user a dark mode or theme.

How does this improve your Mozilla VPN experience?

Reduces eye strain, keeps the application more in line modern applications with regards to customization.

Last request received May 2022 v2.8

┆Issue is synchronized with this Jira Task ┆author: Joseph Anthony Cuevas