WIP: add dark mode

[MightyJoeW]

What: This change adds a dark mode option in the settings dropdown

Why: This dark mode feature will allow users to toggle the UI between the light and dark themes.

How: Add showDarkMode to Settings.js and toggle styles based on settings.darkMode

Pre-Change (no dark mode)	Dark Mode off	Dark mode (WIP)

Checklist:

• Tests
• Ready to be merged
  

Todo:

• Finalize dark mode color scheme
• Extend background height on mobile view (bug)
• In Playground.js, try to setup toggle in className (only working with style tag at the moment)

More info

• usePlayground.js - the SET_SETTINGS type sets darkMode to true with toggle button
• Settings.js - showDarkMode adds the toggle icon
• app.pcss - adds styles for .editor-dark class
• Layout.js and Playground.js - add styles based on showDarkMode

[smeijer]

Closing due to inactivity. Please feel free to reopen when you plan to continue working on this.