Add dynamic Dark Mode Toggle Button

[baymac]

Summary

Currently Dark Mode needs to be added to the gatsby config. We have to hard code the theme. Why not add a dynamic button to switch between themes? As well as use prefers-colour-scheme media query.

Justification

It gives user the choice they want to read the docs.

Desired UX and success metrics

Describe the full user experience for this feature. Also define the metrics by which we can measure success/failure for the user.

Must-have functionality

Are there are any challenges to build this feature? If this discussion has already happen can someone share the thread, I could not find any.

Specific timeline issues / requests

N/A

Available extra resources

N/A