I can't see a way to adjust that.
Instead of using the compiler-generated HTML TOC, you can create your own HTML TOC. Two steps:
- In the Layout section of the compiler, deselect "Generate HTML table of contents."
- Add a Contents document to your project, with the titles of each item you want to include. Format each title as you like, and make each title link to the appropriate folder or text document.
Note: This will affect only the HTML table of contents in the body of your epub file. It will not affect the NCX file that many readers use to build their own navigation links. I cannot see a way to influence what goes into the NCX file other than using folders to indicate titles. (Or editing it by hand afterward.)
Also, I cannot see a way to convince the compiler to generate a hierarchical TOC.
This is one of the reasons I don't use Scrivener to create my epubs. :-( Scrivener is very good at producing nice, clean epub files, but, alas, it does not give me quite enough control over the output.