[Theme: Bombax] display single category on home page when index.php is empty?

With the TwentyEleven theme I had been able to get a single category to show on the home page and all categories on the blog page, doing what is described here:
https://wordpress.org/support/topic/how-to-show-only-one-category-on-front-page

Worked wonderfully! But then I started using Bombax theme and decided I wanted a different category on the home page. That’s where I ran into trouble – the /bombax/index.php file is completely blank.

How do I put the appropriate code in, when there’s no ‘loop’ to put it in front of? Or rather, if it’s in a different file for that theme, in which file would I find ‘the loop’?