Support browser CSS dark mode detection

[33mhz]

Is your feature request related to a problem? Please describe.
Various browsers have started supporting the prefers-color-scheme CSS directive. Beta can take support this.

Describe the solution you'd like
Three settings:

• Dark Mode off: Use light mode
• Dark Mode on: Use dark mode
• Dark Mode not set: use prefers-color-scheme to determine color.

Ref: https://developer.mozilla.org/en-US/docs/Web/CSS/@media/prefers-color-scheme