Add "auto" option for Vela theme

[ackmanx]

Description

I would love to see Vela add an "auto" option to its theme for light/dark.

Value

It would save time to do this automatically and feel more modern. As a bonus to those who use Stylus to enhance Vela's UI it would also prevent theme conflicts.

Definition of Done

When using "auto" then Vela automatically applies the dark theme when the browser detects the system is in a dark theme. Vela automatically applies the light theme when the browser detects the system is in a light theme. If a user manually selects light or dark, then that is used instead.

[chrispdriscoll]

I like this idea. Maybe something we can slide in during a hackathon or if we find ourselves with a little extra capacity. Thanks for the idea.