Sorry I wasn't clear. No, I don't mean the table of contents, or at least not specifically that. I mean the headings as they appear in the document, which - yes - do appear as is in the table of contents.

I'm not talking about editing the style to include a heading somewhere within it, but simply having the comma appear in the appropriate place in the headings as they appear on the page. In other words, the colon would be part of the text of the heading - the part that the style gets attached to. This could be done if I could get rid of the spaces at the end of the style.

As it is now (thanks to your help), if I type the line that will become the heading as ": Introduction", what I'll get when I apply the style to it is "Chapter 1 : Introduction". If I could get rid of the automatic spaces after the style, then I would have "Chapter 1: Introduction", in which only the part appearing in bold face belongs to the style.