Note: If you try that for a cross-site style sheet, bad things happen, so a lot of error handling is needed.
To be honest the question was more out of academic curiosity than anything else because generally speaking using in-line styling is considered bad practice from the perspective of separating layout and presentation hence why we have external style sheets in the first place but I couldn't find a simple straightforward way to access and modify styles in an external style sheet the same way that I can for in-line or internal style sheets, I would have assumed that accessing and modifying styles in an external style sheet would have worked more or less the same way as it does for accessing and modifying styles in an internal style sheet assuming said style sheet is coming from the same place as the rest of the site anyway but apparently not.
Evidently this is why most people don't bother with modifying external style sheets and instead rely on the higher specificity that in-line styling has to override styles when needed as doing that in there scripts is way way simpler.
Edit
so took a look over the page you linked to and like most of these pages they are badly written and about a clear as mud rendering them more or less useless to me.
Honestly I don't know who writes the mozilla developer pages but they clearly need schooling in how to write user-friendly help pages, they also need schooling in accessibility because the font there using is fuzzy, barely legible and hard to read and clearly unfriendly to people with dyslexia.
If I had it my way I'd be telling who ever is responsible to do it again and this time stop using horrible unreadable fonts and actually write there help pages for beginners and not technophiles so that people like me can actual understand what they are talking about, I'd also be telling them to add some simple code example to help readers understand what they are talking about and stop overloading the page with information that is not relevant to the task as hand because it just make it difficult to fine the specific bit of info you are looking for, in this case how to access an external style sheet find a given style rule within it and then modify the value of said rule.