The problem might be the result of a change in the implementation of overriding css files.
The new implementation collects all css files (the default ones and the files you put in the 'web' directory of your project), sorts them alphabetically, and then concatenates them.
As before, if you want one of your files to override a default file, they should have the same name, but the names of the default files have changed to start with 3 digits, in order to ensure they appear first in the alphabetic order (e.g. "000-styles.css", "001-panes.css").
This implies that in order to override "000-styles.css", for example, you should call your file "000-styles.css", not "styles.css". If you have "styles.css", you will get both "000-styles.css" and "styles.css".
To check which files are used by your application, add "/Styles" at the end of its URL (e.g. for the application "App", type in the browser "http://localhost:8080/App/Styles").
For best results, use the Firefox browser..