@ Patrick
RE: I created mycard.css, but these css style definitions will be overwritten by inline style definitions.
Remove the inline styles from you custom template or use the !important modifier in the CSS.
RE: When using the customizer it has no effect. But when customizing the default “card” template, changes has effect in “card” and “mycard”
Best guess, you need to change the const SLUG = 'card'; line in card-default.php to be unique to you custom template. A point not covered in the forum post you found because, I think at the time, the Customizer feature had not yet existed. Or I forgot to mention it. Creating your own custom templates is quite a bit more difficult when compared to using template override files or moving a core template to the plugins folder.
Hope this helps, let me know.
