2024 Theme and user theme support

[svandragt]

I've started on theme support, and have a half baked second theme going in that same branch. (2024, although I'm not sure baking in the date was a good idea. It's probably more like 2010 anyway).

In terms of creating a custom theme, these build on top of the default theme template files as a fallback, so you only need to add the files you want to change, and a stylesheet.

Happy to get any feedback.

[svandragt]

This has now been merged to the main branch and will be part of the 0.5 release, please review the docs: https://svandragt.github.io/lamb/themes.html