Add support for dark and light modes

[vsoch]

We would want to reproduce what we have under https://flux-framework.org. This means the steps to do this would be:

1. Define a light and dark color scheme as is done here. Note that the names there are then referenced int the css.
2. When you have that, you need a trigger to turn it on and off! We have an example include that you could use, here: https://github.com/flux-framework/flux-framework.github.io/blob/main/_includes/dark-light-mode.html
3. The switch itself is added via html here and styling here