Edit: Try looking under
app/assets/stylesheets
Here's an example
https://github.com/joliss/solitr/tree/master/app/assets/stylesheets
I'm not too familiar with the structure of rails apps but did you create a local copy or are you using the bootstrap files being hosted directly by github? You should be able to figure that out by checking one of your launched html pages and viewing the source, looking for something like
<link rel="stylesheet" href="http://twitter.github.com/bootstrap/1.3.0/bootstrap.min.css">
If it's a local page, there should be a directory somewhere in your rails app where the files are stored - perhaps there's a 'static' folder or something similar? Try file-searching for it, good chance you might find it.
(I use Django/Python for web projects but I'll look into Rails a bit and see if I find anything)