CSS not working in stylesheet

This is just a shot in the dark as (at the time of this post) you haven't provided source code.

Make sure you're linking to your stylesheet using a link tag in the head of the HTML document.

If you had:

<style type="text/css">
/* <![CDATA[ */
#someid
{
  margin: 0;
  padding: 3px 12px;
}
/* ]]> */
</style>

You'll need to have

#someid
{
  margin: 0;
  padding: 3px 12px;
}

in your CSS file with:

<link rel="stylesheet" type="text/css" href="path/to/style.css" />

to link to the stylesheet.

Some common newbie mistakes include:

• <style type="text/css" src="path/to/style.css">: because it's a similar syntax to the <script> tag, which would make sense, but is invalid
• <link rel="stylesheet" src="path/to/style.css">: but link elements use href not src
• placing link elements within the body: although browsers will tend to manage link elements in the body, there are likely going to be some errors, and it's not a defined behavior
• not specifying a doctype declaration: allows the browser to go into quirks mode, which is never a good idea.

You should make sure the stylesheet is properly imported. Sometimes the @import doesn't work well if not used accordingly, so always reference your stylesheet:

<link rel="stylesheet" type="text/css" href="name-of-stylesheet.css" />

Always remember to close the <link> tag as it's a self-close tag. I think @zzzzBov forgot to mention that.

Finally, if that doesn't work, try to override some of the styles by physically writing (above the </head> section) something like:

<style type="text/css">
    body { background: blue; }
    * { color: red; }
</style>

and see if that gives you a blue background and red colored text. It should. After that, try to implement the referencing method and make sure you reference the stylesheet file to the right directory.

Good luck!