implement default Nextcloud branding

[SunboX]

• use the nextcloud blue as accent color (default)
• prefer the color from the theming option on the server
• make this configurable in the settings
  • so the user can decide if he wants to keep the nextcloud blue (or the theming option on the server) or if wants to use the system defined accent color
• use color extensively, see screens below (first one is new, second one is current state)
• screens also address horizontal spacing

See https://github.com/nextcloud/windows-universal/issues/18#issuecomment-260318625