In continuation with my post about SharePoint Mapped folders, Lets look at an example for how to add or deploy a custom css file in SharePoint 2010’s _layouts/1033/Styles folder.
1. Open the Visual Studio and Create a Project “DeployCustomCss” file.
2. Deploy it as a Farm solution.
3. Once done right click on the Project and add navigate to “Add -> SharePoint Mapped Folder”. A list of 14 hive folders will open up in a popup window. See the screen below
5. Once you select it you would see the newly created STYLES folder in your solution explorer.
6. Next, simply add your CSs file in the folder and Build & Deploy.
7. Navigate to the C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\14\TEMPLATE\LAYOUTS\1033\STYLES to check your CSs file.