Kinda the point of the generic content wrapper is that it's, well, generic.  We use it in loads of different places so the CSS only has to be specified once with any theme, and it'll work everywhere.

If you need different CSS for seen and unseen, you probably want to set the id rather than the class, so this line should probably be something like <div class="generic-content-wrapper" id="mail-list-wrapper{{if $seen}}-seen{{/if}}{{if $unseen}}-unseen{{/if}}">

If you did it that way, you'd need to check it with Redbasic, but this template could probably go back upstream.