The CSS, and a bookmarklet to apply it are available here:
https://gist.github.com/douglas-johnson/aa7992550591896ec0c78615dea60341

You can use them to see the stylesheet applied using a desktop browser.

1. Create a bookmark with the bookmarklet text as the URL. It starts with javascript: rather than https:
2. Navigate to a forum or topic on the board.
3. Click the bookmark with the custom javascript in it.

This will fetch the CSS from GitHub and append it as a style element in the head.

I wanted to see how far I could get with just CSS, and now I know.

The limitation I'm hitting is the table of contents and nested comments. The indentation is handled with literal whitespace like non-breaking spaces, which CSS doesn't have much control over.

A possible solution would be more JavaScript to produce a character count that let's the CSS know how much indentation to apply. I'm not sure I can make it terse enough to fit it in a bookmarklet.