Does anyone know which theme is used on this site (Nordic Open Access Scholarly Publishing) and is it open source?
If you go to View Page Source (or comparable option, depending on which browser you’re using, and look at the HTML page for a journal and do a Ctrl + F (Command + F on Mac) for “theme” you’re likely able to identify a theme that is being used. In the case of this journal - one sees references to “default” - likely meaning that it is based on the default theme, however, in another instance where “theme” appears it also reads “noasp” which means that the theme is likely custom developed by this particular organization, so they have invested some resources into customization. So far as I can tell, they don’t have a Github page and have not made the source code available for their particular theme. If you’re interested in customizing themes, I’d encourage you to have a look at the theming documentation: PKP Theming Guide