[deback]

In the Editor, you only need to change the left and right margins for the body class (in the *.css file), which is normally "calibre," but not always. If you change the body class and then change the left and right margins in the classes for the paragraphs, you will not have the results you intended.

Another way to change any of the four margins is to use Convert. Open the preferences from the main Calibre window (ctrl-P). Click on Common Options and then click on Page Setup. Over near the bottom on the right, fill in the left and right margin lines with the values you prefer. Click Apply and then when you convert, your margins will be what you've set under Page Setup--and you won't need to edit the page_styles.css or the stylesheet.css files.