Instead of passing plain CSS to Odoo, you can also make use of less (http://lesscss.org), which is a higher-level notation for CSS that helps you write more efficient CSS code. Certainly for big code bases, it will improve maintainability a lot. Given Odoo uses this internally anyway, you can rely on it being available. So, to use less instead of plain CSS, just point to the less file in your link element and use type="text/less"; this will make Odoo run lessc on your file in order to generate CSS on the fly without you having to do anything for it.