CSS is not working properly

I added the code given below in Stylesheet (style.css) file.

code{
background: #eee;
}

So that whenever I use <code> in my post it will get a background color by default.
While adding a new post I put <code> Here is my code.</code>but I see no background changes.

If I use

<span style="background: #eee;">Here is my code </span> direct in the post then it works.

Why isn’t it working when I use <code>?