Here's a real world example with the use of non-minified versus minified assets in your layout.
Web.Config
<appSettings>
<add key="Environment" value="Dev" />
</appSettings>
Razor Template - use that var above like this:
@if (System.Configuration.ConfigurationManager.AppSettings["Environment"] == "Dev")
{
<link type="text/css" rel="stylesheet" href="@Url.Content("~/Content/styles/theme.css" )">
}else{
<link type="text/css" rel="stylesheet" href="@Url.Content("~/Content/styles/blue_theme.min.css" )">
}