Hello Andrew Ryan,
You can create a custom control that derives from GenericContent class. Then you have to create a public property in the GenericContent designer where you will enter the text for your content, since the RadEditor content area will be used to enter the content. You can add the css to the ContentPlaceHolder of the RadEditor control or just ApplyCss to the RadEditor content.
In your custom designer
you have to create a property which will be used to set the Title. Here you have to override InitializeControls method which is uses to get the data when you open the designer.
Title.Text = ((CustomGenericContent)DesignedControl).GenericContentTextBox.Text;
You also have to override OnSaving() to persist the property value.
In the custom control that derives from GenericContent
you have to override Render method and set the GenericContentTextBox.Text to the text you have entered in the designer.
You can use shared content as well which will allow you to work with metadata. You may find the following KB useful - Wrap Generic Content control in Sitefinity 3.6
( the same applies for 3.7)
the Telerik team