ASP.NET : Change CSS from code behind.

A student/colleague of mine, after checking out a site I did, asked me: how do you change CSS files dynamically?

The answer is very simple;

I’m using a master page here and usually in the head container there’d be your css link;

By adding ‘runat=”server”’ and giving it a unique id you make the link run server side, and then after that modifying the ‘href’ attribute is done in the code behind.

<link id="masterCss" href="App_Themes/Theme1/yourcssfile.css" type="text/css" rel="stylesheet" runat="server" />

Then in the code behind :

masterCss.Href = "App_Themes/Theme1/secondcssfile.css";

Clearly that would be best set up in : “Page_Load” event or “Page_PreRender” depending on your needs :).