In addition to previous replies:
Regarding official themes take a look at this post: Layout Modification - #2 by Vitaliy
In the future PKP team is planning to release more. All of them are free to use and share the same license as OJS.
Also there are several themes developed by a community. E.g., my experimental theme: GitHub - Vitaliy-1/oldGregg: Theme for OJS 3 with Bootstrap 4, it is not official and will be a subject for changes in the future.