Here are the steps to use your own css for your Odoo website: https://drive.google.com/file/d/0B1uIL9E_zXrrMVRUeGlQcW9rN3M/view?usp=sharing

Here is the code for the new view:
<?xml version="1.0"?>
<data inherit_id="website.assets_frontend" name="mycss">
      <xpath expr="." position="inside">
          <style>
              INSERT YOUR CSS HERE
          </style>
      </xpath>
    </data>

You can also upload your css file into Odoo and use it: https://drive.google.com/file/d/0B1uIL9E_zXrrMmVoa0QxVjV3aGs/view?usp=sharing

Here is the code for the view:
<?xml version="1.0"?>
<data inherit_id="website.assets_frontend" name="mycss">
      <xpath expr="." position="inside">
          <link rel="stylesheet" href="INSERT YOUR CSS SHEET LINK HERE"/>
      </xpath>
    </data>