For most things, the easiest is to just add
<div id='back'> at the very top of the back template, and a corresponding
</div> at the very end of the back template. Then after you have styled the front side, you can apply style overrides for the answer side using the
Background is a little different (and so I am starting to concede the original point in this thread). If you refer back to my screenshot, you will see that there is a
<style> block directly under the highlighted line. That is how Anki is inserting your template CSS. Well, you can do the very same thing: you can add your own additional
<style> block directly in the back template—not the Styling section—like so:
No need to do anything special to the front of a card: just style the front as default and target the back for any differences. But do remember that if a note type generates more than one card type (such as reverse), you will need to repeat the things you have added in each back template.
I have to admit, any CSS that you want to apply directly to
.card, such as background, would be easier if it were differentiated by class.