Add option for visitors to switch to dark mode

[theecrit]

What are you submitting?

Place an x in the appropriate bracket:

• Bug (something's broken)
• Feature request or improvement
• Content update
• Question
• Other

Description

Received a request for the site to offer a dark mode version. This issue is a placeholder that needs definition.

Considerations

• UI discoverability
• Accessibility (should color scheme be addressed within a larger context?)

Thank you!

• Please document your design and tech decisions in your pull request so others can follow along and understand why we've made the decisions we have.
• If you'd like feedback, input, or help, share your thoughts or question in this issue's comments.