Writing help files for an Eclipse RCP is easy. Help content is simply provided as normal html files. And of course it is possible to format the html code using css. The problem now is, that help view and help browser use the same css, but the view is of course much smaller.
To solve this you need to provide a plugin_customization.ini (defined as a product property in your plugin.xml). Provide at least two parameters for your help files here:
topic_css is used for normal help pages, narrow_css only for the help view or help tray.
There are two more parameters available, see the Product customization help page for more information about them.
Another benefit is that all your help pages will look the same. Simply do not integrate any other css style (or make sure you do not overwrite the injected styles) and wait for the help runtime to inject the matching css styles.