It contains a theme coded specifically for the blog, with examples taken from Hugo documentation. The purpose of not taking a great existing theme is to learn new webthings (because I really suck at web development). Also I wanted to learn a bit about Bulma, a tiny CSS framework without a line of JavaScript.
Hugo is really powerful yet pretty light and really fast. I do like the way templating is used to achieve very elegant tricks. In the past we would have used databases and dynamic languages like PHP for the same result.
I think that using Wordpress for a little static blog is no longer relevant and I am really enthousiastic about projects like Hugo. Additionnally, the community seems very friendly. Other static/flatfile CMS such as Ghost have an economic model that I don't like (eg you have to pay for plugins) and Grav is great but not so artisanal : the learning curve is pretty high and you stll need PHP.
TBH I haven't heard of Zola when starting this blog. Maybe today I would choose Zola, because it seems less complex and making more sense (eg with its templating language), but I'm still happy with my theme.