Hello, Yewon,
Thanks for your offer of help. I think there is something you can do to help, if you're interested, but it's a bit more complicated than changing the CSS styling.
The Korean translation looks the way it does because it used to be hosted on the translator's personal website, which went down in 2019. I rescued it using the Wayback Machine and added it to the Beuatiful Soup code repository.
The original English documentation and the other translations are stored in reStructured Text format, which makes it easy to create versions in different formats. It's clear from the formatting that the Korean translation was also originally stored as reStructured Text, but I was never able to contact the translator to get a copy of it. I was only able to recover the bare HTML and a basic CSS file.
If you could convert the existing Korean translation back to reStructured Text, it'd be on the same level as the other translations. This would have two advantages:
1. The HTML version of the Korean translation would have the same styling as the other translations, because it would be built by the same tools. It would also have improved presentation features like an automatically generated table of contents.
2. It would become practical to update the Korean translation.
The second one is the really important one for me, because the Korean translation is dated November 2012. It should still be accurate, but so many things have been added to Beautiful Soup in the last thirteen years, that I think it's less useful to a Korean-speaking developer than a machine translation of the current English documentation. But it's not reasonable to expect someone to update the Korean translation when they'd have to directly edit an HTML file.
If you want to modify the index.css file currently used with the Korean translation, I'll accept the contribution and update the repository, but I think the styling is just a symptom of the translation's complicated history. I think it'd be healthier for the future of the Korean translation if you or someone else could reverse-engineer a reStructured Text version of the HTML file.
Leonard