Calibre shouldn't ignore stylesheet changes you made in Sigil. There's probably something else going on. If you can create a reproduction case with some generic non-copyright content you could post it here, otherwise post it to a bug with the file on bugs.calibre-ebook.com.
You did actually change the beginning of each of those paragraphs to <p class="calibre7">, right? One thing I probably wouldn't do is use the name calibre7 or any calibre name in my own css styles. Create an name like 'noindent' or something like that. I don't think calibre actually reserves those names but it's possible that's tripping things up.