Re: Where are this theme's files coming from?
That 1col_fixed theme only has front-page.php, header.php, footer.php, and a few other miscellaenous files.
If bbPress can’t find a particular template file, it will pull the ones from the default theme, Kakumei (stored in /bb-templates/kakumei )
More here: https://bbpress.org/documentation/themes/
I would copy the missing files from kakumei over to your 1col_fixed theme, and then you can edit them right there. Good luck!