Yes, AO3 has a feature for authors to make custom 'work skins'; basically including custom CSS for their stories. AO3 also lets you create & use custom skins for the site itself, but that's distinct and for the
user vs the
author.
I know that I looked at it in detail before (and even used it for a story), but elected not to include an FFF feature for it. For... reasons?
I probably judged it too much effort for too little reward. I think most authors generate their HTML with style attrs instead of custom CSS. At least, I don't remember seeing very many stories that use it.
FFF's customization features should allow you to apply it for a given story anyway with relatively low effort.
Search the story source HTML for "workskin" and you'll find a style tag with the work skin CSS you can copy.
Depending on what's in the CSS and how it's applied, you probably need to either remove the #workskin selectors from the CSS or add the id="workskin" attr to your chapter HTML appropriately. That second could be tricky? Oh, and escape any % to %% for INI.
This appears to be sufficient for that story, copy paste, find/replace, escape %, done:
Code:
[https://archiveofourown.org/works/36423070]
add_to_output_css:
p{font-family:'Helvetica',sans-serif;margin-top:7px;margin-bottom:7px;margin-right:15px;margin-left:15px}.chat{width:350px;margin:auto}.leftlabel{color:#9f9fa0;font-size:11px;font-weight:bold;margin-bottom:0;float:left;margin-left:0}.rightlabel{color:#9f9fa0;font-size:11px;font-weight:bold;margin-bottom:0;float:right;margin-right:0}.centerlabel{color:#9f9fa0;font-size:11px;text-align:center}.line{display:inline-block;width:100%%}.message{font-size:16px;max-width:300;overflow-wrap:break-word;margin-bottom:10px;display:block}.sent{border-radius:30px 30px 5px 30px;background:#0b82ff;color:#fff;float:right}.received{border-radius:30px 30px 30px 5px;background:#e5e4ea;color:#000;float:left}