Are these fields that have failed a scripted validation rule or just those that are required but not filled in?  Does this both affect system pages and custom pages?

These fields are fields that are required but were not filled in. It affects mostly custom workflow pages where there is no custom content field. With a custom content field I could at least show which fields are required by changing their background color but as there is no such field I had hoped to have a 'global' solution that works on every screen.

If you are happy to go down the route of a custom script then you can look to inject script into those custom pages by way of a CreateScript or translations.  I'm not sure if it is still strictly supported but it works.

Thank you for the suggestion, that works like a charm! For the record, i found this 10 year old tutorial and implemented the highlighting of the required fields accordingly: https://www.sagecity.com/sage-global-solutions/sage-crm/b/sage-crm-hints-tips-and-tricks/posts/adding-clientside-code-into-dynamically-built-workflow-action-screens

That should at least mitigate the problem of fields not being filled in for now

The old ones are the best.