CSS Selectors on page load

I have the following code to get the current URI and then a conditional to say "if" then perform a certain action. How would I apply a certain CSS style to an element ID? I've read something about using runat="server" on the element ID I want to change, but I can't find anything do assist with CSS selectors.
@{ string CurrentUrl = Request.Url.AbsoluteUri; } @if (CurrentUrl.Contains("3d-printer")) { //CSS Condition goes here }