I figured out how to use OJS 3.0.2 and how to implement a personal theme. Everything was awesome.
Today, I tested 3.1, installed my theme as described in the “bootstrap3” repository, and enabled the theme-plugin in the administration. However, although enabled, OJS does not show my theme in the dropdown menu in the Appearance tab. I also tested this with the bootstrap3 theme. Same result!
With OJS 3.0.2 both my own theme and bootstrap3 worked just fine.
Can I manually change the theme somewhere in the configs?
OJS runs on a Suse Linux with PHP 7, if this is important.