The snippet I posted should do the trick, just change the border radius property under .callouts-outlined .callout .callout-title from border-top-left-radius: 10px; border-top-right-radius: 10px; to border-radius: 5px;
You do need the style settings plugin though to get it to work with Minimal since my CSS builds off of the ‘Outlined’ style callouts option off there.
The main thing with CSS is finding the right selectors for the elements you want to change. Here’s the Mozilla documentation on that. This post goes over the basics of using the developer console to find the right selectors.