Download ranobe from ranobelib.me as archived zip.
To install this sctipt you need browser addon such as Tampermonkey https://www.tampermonkey.net/
This script will add new button on title pages at bottom-right with download icon:
Script will save zip archive with folders for text chapters (.txt) and html chapters with styles and buttons to navigate (very simple). HTML file also use dark theme.
To see progress, you may open console (Ctrl+Shift+I) and watch download logs.
You can find similar userscript for mangalib.me