Feature: detect dark mode preference from device.

[MarkHalls]

Summary

We should set the default display mode of the app to the users device preferences.

Objectives

• on initial load, dark/light mode is set from user device preference.

Mockups

• https://www.figma.com/file/SmslveGClPv8Rk5gTKsAOZ/Trash-Panda%2C-Lynn-B.-Kendra-M.?node-id=925%3A405