Cool part about SP2010 is that as soon as it sees styles in the custom CSS file, it will add it to the Style dropdown, I made a quick documentation for a colleague so I thought would blog it as well.
You see the “Styles” in the Ribbon that is where I want to add my custom styles
This is a 2 step process:
1. All the custom styles in the custom css file should has the prefix .ms-rtestyle-
-ms-name:"My Custom Header";
2. There are 2 ways to attach you Custom CSS file to the masterpage
Option 1 : Either specify the custom CSS file as an alternateCSS file by gong to CA\Site Settings\ Look and Feel\ Master Page ( you will have to activate the publishing feature for this option to be available)
Option 2 : Just add a Link to the CSS file in the master page