I was looking around to get a feel for the changes when I noticed that the colour scheme is different on some pages, I'm not sure why?

Red and black (darkmode) https://developer.mozilla.org/en-US/docs/Web/HTML/Element/bo...

Blue and black (darkmode) https://developer.mozilla.org/en-US/docs/Web/Guide/HTML/Cont...

Both pages are under the "References" section.