Out of the box or provided by someone else. As an example, Hugo has these: https://themes.gohugo.io/
Related question to those who built your sites using static site generators: How did you find the theme for your site? Did you go with what is available? Or did you design it yourself?
Personally I built a simple theme from scratch, but it really depends on what your use case is. A personal blog is different than a product landing page. Short of looking appalling though, I would recommend focusing on the content of the page instead. If the content is good you'll get eyeballs regardless of the looks, not so much so the other way around.
👤 dcminter
I use Hugo which has a themes gallery [0], which I assume is common (Edit: and as you already noted above - I should read more carefully!). I perused that for something acceptable and then tweaked it a bit. The result [1] could be better but is good enough for my purposes.