The default css theme
The default css theme is not mobile friendly.
The default css file is pretty much useless anyway. It is there only as an example. I always write my own from scratch. It could be nice to include some better looking themes (especially for syntax highlighting), but I think that misses the point of Hakyll.
Some people port themes over from Jekyll, where there is a more plug-and-play culture for static blogs. That works great when you just want to get a nice looking blog online, but it is a nightmare to do any customization. Hakyll focuses on easy customization. So I don't believe it is a good use of anyone's time to create a fancy default css file that will just be discarded anyway. If you really want to do it, by all means, go ahead and submit a pull request.
Yeah, although many users are not really interested in writing CSS AFAIK. I wouldn't mind having the option to choose a theme when you run hakyll-init.