You have the great ability with mautic to create themes that helps you to create great emails that matches to your company guidelines.
Nevertheless, when you create your theme and then add a new CTA button on the email drag and drop builder, the button style is not following the theme directives but the mautic core button style that are inline style.
Inline style is stronger than general style, especially in emails
Consequently, it is simply impossible to restyle an button which was not previously planned in the template —> the drag and drop feature becomes useless.
With a config file in the theme structure, we could allow theme developers to redefine the standardization of slots (zones) to drag and drop.
That way, instead of a standard mautic button, you could drag and drop a theme styled button and take benefit of the power of drag and drop email UI !