Enable dark theme

[prabuw]

High-level description of the approach to enable dark theme:

• Add a react component to capture the theme state.
• Add react-context that to allow the passing of the theme state between the button that toggles the theme and the component that holds the theme state.
• Add _app.js (nextjs docs) to control page initialisation so that the front-end state (e.g. theme choice) persists across pages.
• Toggle a CSS class (dark-theme) on the body element that triggers the dark theme.
• Add SASS variables for dark theme colours
• Add styles to components that use the dark-theme CSS class to override the normal styles.

Things considered but not implemented

• Store the user's theming choice to local storage so it is remembered across visits to the site when using the same client and device.

Closes #297.

Screenshots

[prabuw]

@pastelsky Hello, there was a GitHub issue for implementing a dark theme but I can't seem to find/access it anymore. I wanted to attach this PR to it. It is issue #297 I believe with a label of good first issue.

Strangely, GitHub shows there are 3 open issues for this label but I can't see them when I click through.

Do you have any idea what has happened? 🤔 😅

[VivekNeel]

Awesome, can't wait to get this merged.

[piofinn]

Is there any reason why CSS variables aren't used? Seems like it would reduce both the amount and complexity of the code by a lot, and I highly doubt any users of this page use an unsupported browser 😅

Support for global (OS-level) theme and/or remembering the user setting on the page would also be really nice. I'll be happy to assist with any of these if needed! 😊