As part of my custom module I've created a .css file, which is added to the manifest under the QWeb section and is then added to the report template using the following XML:

most of the time this works; however recently I start to see the following error when starting the application:

ERROR Test odoo.addons.web.controllers.main: Could not parse file /mnt/extra-addons/module/static/css/styles.css: Start tag expected, '

if I then remove the module from the addons folder i can restart the server; subsequenlty copying the module back in and reinstalling it allows everything to work again.

Other than adding the css directly into the XML rather than the link to an external file; is there a more correct way to add custom css into a module?