As far as our CSS styles grow I found that there should intuitive way to manage CSS inclusions. I’ve decided to follow great Grails fundamental idea and place them by some convention and include automagically.
We found useful to have separate CSS for each view (or controller action if you wish). Convention is simple: place each CSS in the same path as you place your views under /views folder.
For example, we have view: /views/picture/gallery.gsp and want to have gallery.css which contains all styles for this view. We should place it in the same path as our view located, in our example it should be /web-app/css/picture/gallery.css. Now to automatically include such CSSs by convention we should modify our main layout (/views/layouts/main.gsp) next way:
<head>
...
<% if (webRequest.controllerName && webRequest.actionName) { %>
<link rel="stylesheet" href="${resource(dir: 'css/' + webRequest.getControllerName(), file: webRequest.getActionName() + '.css')}"/>
<% } %>
...
</head>
All further CSSs placed by this convention will be automatically included for each action (which uses main layout) . If you have more layouts you could modify them same way. Anyway their count will be much less than your actions/views count.
Hope this helps. If you have some ideas to improve this approach or you use some other approach, please share :)