I'm using bootswatch themes, and I downloaded the Journal theme (on the Navbar click on Journal, there you'll see the download files), and I have the option to download a bootstrap.css file or a bootstrap.min.css (check out here to see what the bootstrap.min.css means, but essentially it's the same thing as bootstrap.css), so I downloaded the bootstrap.css file, and indeed the whole look-and-feel of my website looks like the Journal theme.
But when I first created my visual-studio project, I got in my Content folder, also a bootstrap-theme.css file (see here what this file is), and this file isn't available when downloading from Bootswatch, so I wanted to know can I use any theme with this bootstrap-theme.css or does every theme have a bootstrap-theme.css of its own? Because I don't see any bootstrap-theme.css when downloading a theme from Bootswatch.
bootstrap-theme.css is a very old add-on theme specific to Bootstrap 3.x. It's no longer relevant, not used for any Bootswatch themes and should be removed from the project.