Toonlight

WEBTOON's website has no dark mode. This makes webtoons.com dark: the menus, series pages and episode lists, the reader, comments and the mobile site.

Unlike general dark-mode tools, it doesn't guess or invert colors. WEBTOON's pages were styled by hand, so the comics stay exactly as they were drawn. An optional night dim lowers only the panels' brightness when you read in the dark, and keyboard shortcuts turn the theme and the dim on and off.

It's free, with no ads, no paid version and no account. It collects no data and runs only on webtoons.com. The code is open source (MIT).

Downsides: it only works on WEBTOON, and when WEBTOON changes its site, some parts can show up light until the theme is updated.

Also available as a userscript for Tampermonkey and Violentmonkey, on Greasy Fork. An independent project, not affiliated with NAVER WEBTOON.