Since all pages in OJS share the same common header and footer templates, you'd need to do some fairly serious hacking in there to override the CSS based on the current page. To do it cleanly, you'd probably want to go into the various page handlers (the PHP code) and set some new template variables that would only exist within the editor interface. So, probably the sectionEditor/, manager/, reviewer/, workflow/ pages (for example). You could test for those variables in the Smarty templates and modify your design accordingly.
Alternatively, write a plugin that Hooks into some of the request or template manager calls so you can examine the page or the current op and maybe load a custom style sheet that makes the changes you want. That way you could leave the rest of OJS' handlers unmodified.
Just a few ideas.
Software Developer, Public Knowledge Project Team