The CSS subtab contains the CSS files that Oxygen XML Author Eclipse plugin uses to render a document in the Author mode. In this subtab, you can set main and alternate CSS files. When you are editing a document in the Author mode, you can switch between these CSS files from the Styles drop-down menu on the Author Styles toolbar.
To open the CSS subtab, open the Preferences dialog box , go to Document Type Association, use the New, Edit, Duplicate, or Extend button, click on the Author tab, and then the CSS subtab.
The following actions are available in the CSS subtab:
New- Opens a dialog box that allows you to add a CSS file. You can
specify the path by using the text field, its history drop-down, the
Insert Editor
Variables button, or the browsing actions in the 
Browse drop-down list. 
Edit- Opens a dialog box that allows you to edit the current selection.
Delete- Deletes the currently selected CSS file.
Move
Up- Moves the selected CSS file up in the list.
Move
Down- Moves the selected CSS file down in the list.
- Enable multiple selection of alternate CSSs
- Allows users to apply multiple alternate styles, as layers, over the main CSS style. This option is selected by default for DITA document types.
- If there are CSSs specified in the document then
- You can choose between the following options for controlling how the CSS files that
are set in this subtab will be handled if a CSS is specified in the document itself:
- Ignore CSSs from the associated document type - The CSS files set in this CSS subtab are overwritten by the CSS files specified in the document itself.
- Merge them with CSSs from the associated document type - The CSS files set in this CSS subtab are merged with the CSS files specified in the document itself.