You can change the color, font properties, stroke color and size, etc. and then save your modified callout for reuse. But if you want something drastically different, you'll have to create your own. CMAC supports pdf files, so if you make a vector drawing in pdf it should scale nicely. We also support a wide variety of image formats so you can also generate your callout in an image editor and bring that in. Note that raster images don't scale as nicely as vector images, so you'd want to make the callout the size you want it to be to get a really crisp look. But you won't be able to change the text or any other properties of those images you bring in. You could create an image that just has the background look you want and then place one of our text only callouts over top of it. The text wouldn't automatically wrap or scale the surrounding callout, so you'd have to do that manually. But once you have it looking the way you want, you could then group the background and the text together so you could manipulate that as one object for positioning and moving around the canvas as needed.