Sorry, there are no options in Markua for formatting the Table of Contents.

This is exactly the type of thing that we want authors to not worry about or spend any time on. So, we're opinionated here.

I know exactly how you feel -- I've written technical books too, and if there was a workaround, I would have used it too, and I would have noticed, just as you have. However, it provides essentially no value to your readers, and it is a slippery slope: from this, it leads to mucking around with image positioning, widows and orphans, margins, fonts, etc, etc, and it's a great way to procrastinate instead of writing :)
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Please note that we do offer some customization if you select a Custom book theme by going here:

...making sure to replace YOUR_BOOK with your book's unique web address.

You can see what the Custom options are if you scroll down here:

http://help.leanpub.com/en/articles/3691789-what-are-the-settings-for-each-of-the-different-book-themes

...and look for the Table of Contents section:

Now, all that being said, there are some very big hammer workarounds:

1. Change the page size here (the most options are in the Custom style):

2. When you are 100% done the book, export InDesign and have a designer make a beautiful book:

Neither of these are things I'd recommend while a book is in-progress, of course :)