Sure; Portal themes run as (or inside) a typical Java EE WAR. There's no reason your custom Spring-based web apps can't directly reference your Portal's CSS files.
You shouldn't have to copy the CSS selectors, you should be able to reference your Portal's theme CSS files directly:
<link type="text/css" rel="stylesheet" media="all" href="/portal/yourportaltheme/styles/style.css" />
The above should work if your custom spring apps were deployed to the Portal server; otherwise you'd want to reference the appropriate server, etc.
You can't, however, easily take advantage of the other features provided by Portal themes - layouts, regions, etc.
Along those lines, if you do deploy the Spring apps on the Portal server, you should easily be able to enable single-sign on between them: http://www.andypemberton.com/jboss/securing-ajax-servlets-in-jboss-portal/