add
Adds custom CSS to the WebView and returns a handle that can be used to remove it again.
The CSS is inserted as a <style> tag in the top-level HTML that contains the ReaderView, Renderers and ReaderViewAnnotationLayers.
The CSS added with this method will NOT affect the ReaderPublication styling. For adding custom CSS to EpubReaderPublications, see EpubReaderPublication.options.customPublicationCss
This method can be used for styling ReaderViewAnnotations. See ReaderViewAnnotationLayer for more information.
Return
an opaque handle for the added CSS block, to be passed to removeCustomCss. Handles are meaningful only to this ReaderView.