Copy the code (using the dots to the right of the class which is actually a menu window activator) and choose copy to and choose your custom CSS file. Now edit that to your hearts content. Nothing is truly "locked", it's just locked to directly edit it is all.
No, you just need to copy that CSS rule so you can override it. In the app click on stacked triple dots and select "Copy to" and select styles.css then you can edit that code to override the BSS default.