I looked for a pop-up banner to inform users about a service outage due to scheduled maintenance. Then I used the official PKP’s Custom Header Plugin and I added the following code (freely inspired by this webpage, found on google).
Basically it adds a gray background and a centered box with your message. Both are not visible as long as the title-page contains a pipe (i.e. “Submissions | Journal Title”). For my experience, this condition should make it pop-up just in the journal homepage.
If you are not an expert user, you can paste the whole text in the plugin settings replacing only “This is an alert about my journal”. Otherwise you can customize it with css and html instructions.
It’s just a unsophisticated solution, so any suggestion is welcome!
I’m happy you find my solution useful! Honestly I can’t reply to your second answer. In the first post you asked generically for a pop-up box for textual messages. The cookie management is more complex.
I’m not affiliated in any way to that site, so read their page and value by yourself. To me, they works.