I believe 'redbasic:dark' as the Comanche theme name might provide better results. You should be able to use a theme:scheme pair anywhere you can use a theme name.
They may indeed do something with .scss files but I'm not really familiar with Foundation and cannot advise on how they do things. Still, I think it is more the Hubzilla side you are struggling with and probably not so much Foundation.]
You can write your own css or include foundation or bootstrap with the [css]foundation[/css] tag and use their classes.
F6 states that there is a _settings.scss file in a src folder with every project, where you can customize apprearence and colors. But in Hubzilla, I cannot find this file. And I do not want / know whether I may mess around in the regular files.
We use it to provide variable names in CSS files. This is currently implemented (as an example interface) in view/theme/redbasic/php/config.php. You can extend this configurability to new themes if you use the same structure.
[template]minimal[/template][theme]redbasic:dark[/theme][comment]Adding a HTML HEAD region to the Default template and select Foundation by Zurb for CSS and JS.[/comment][region=htmlhead] [JS]jquery[/JS] [CSS]foundation[/CSS] [JS]foundation[/JS][/region][region=content] [block]some_block[/block] $content[/region]