Shaarli is a great piece of software, but does seem to lack a bit in the variety of themes available for it. After upgrading, I found that the theme I'd installed that was a clone of the Delicio.us theme actually caused the site to 500, so I went looking for an alternative. This theme looks serious! Lots of commits, and based on the screenshots, it's very faithfully sticking to material design.
Sadly, while I've installed it on this instance, I'm still getting errors when I try to enable it, so I'll have to dig in and see if that's something with my instance or if I should look at updating the theme. This instance is using version 0.12.0 of Shaarli, but the latest release for the theme is 0.11.0.