In Backdrop, as in Drupal, there usually seems to be more than one way of doing the same thing, and in this blog contribution I am not suggesting that this is THE way to theme your site, it just happens to be a way I have used with some success.
I do not consider myself a coder. My contributions in the issue queues both in drupal.org as well as in the GitHub repository of Backdrop CMS had very little to do with actual code. That was until a few months ago Geoff (a.k.a. @serundeputy) showed me a way to create PRs though the GitHub UI that I find very easy (although not very coder-like).