Mimicking a Rollover, even in Fluid Boxes, is possible if you are sure lot of your learners are using desktop/laptop. Have a look at these workarounds:

http://blog.lilybiri.com/rollover-in-fluid-boxes

Check out this great article by Kevin A. Siegel, author of Adobe Captivate 2019: Beyond the Essentials. This article provides a great workaround that will work with HTML5 on desktop computers.

https://blog.iconlogic.com/weblog/2020/01/adobe-captivate-2019-an-alternative-to-rollover-captions-and-rollover-images.html

I never use rollover captions anymore but a multistate object or a shape defined as rollover. Hope you are aware that those rollovers will not work on mobile devices? They are also mentioned as incompatible with HTML5 output by the Tracker, but will be functional for desktop/laptops even in HTML output.

To answer your question: in most recent Themes the ‘old’ caption as text container is only used with a transparent box. But you can always choose another Caption style in the style field. Defining custom caption styles is bit more complicated because in almost all cases they can be replaced by a shape, for which styling is lot easier and can completely be done within Captivate. For captions you’ll need a graphics application.