What are themes?
There are several visual appearance out there for Bootstrap, which can be found from sources, such as Bootswatch, which are modifying the bootstrap.min.css file. You can also create your own theme this way.
When to modify themes and when to add new rules to a site.css file?
When to modify the
file, and when to add your own .css file, such as
Sometimes there are style requirements, which must be done, no matter what theme you are using. These rules should go into your own .css file, such as site.css, so the main theme can be changed, the rules from site.css will apply anyhow. In order to do that, you just have to link the bootstrap theme, and your own rules, to override the existing ones:
<link href="../Content/bootstrap.min.css" rel="stylesheet"> <link href="../Content/site.css" rel="stylesheet">
This way, Bootstrap themes can be changed anytime without losing the mandatory rules, applied from site.css.