That's the HTML that makes up the text of the book. The CSS is in the stylesheet.css file noted at the top of what you posted. In the Calibre editor, you should see that in the left column.
But, if you're seeing a narrow column of text on the ereader and a normal width in Calibre's editor/viewer, it's possible you've set your margins on the Libre too big. On the Kobo, in the book, if you bring up the reading menu and look at the font section, is your margin slider all the way to the left? If not, move it there. If it is, copy that CSS from stylesheet.css here.
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