either you're kidding... very good... or your ISP is doing that... but wait that's not possible for https content is it? I don't know, nothing's implausible in this world anymore.
It's more likely that he has a browser extension that is doing this. I have DarkReader installed, and I similarly saw a bunch of style tags at the beginning of the page related to DarkReader (which applies some CSS rules to make everything "dark mode"). To the GP, you probably want to double-check your browser extensions (unless you were actually kidding).
FWIW I don’t see anything extraneous, neither in the source code nor on the page. Browser extensions can inject content, of course; that might be what’s happening here.
Yes, HTTPS can be man in the middled by your ISP.
Only if they have you trust their certificate root, or you constantly click through the certificate warnings.
This seems mostly not true. Can you elaborate how?