I'm sorry to hear about the passing of the project's founder. You have my condolences.

Two off-the-cuff pieces of feedback:

1) The PDF link is broken.

2) You can achieve a Light/Dark theme toggle using pure CSS, using a combination of prefers-color-scheme[1] and/or a combinator selector (~) and checkbox. This could do away with the JS entirely.

[1]: https://developer.mozilla.org/en-US/docs/Web/CSS/@media/pref...