Scan through all theme files when searching for commerce_invoice_receipt.css (for 2.x branch)

Hello,

Right now the module forces themes to override the invoice stylesheet in one of the following locations: "/invoice" and "/css".
I think that this is a bit counter-intuitive, especially as it is not documented anywhere.

Often themes have special folders where overrides should be placed; e.g. "css/overrides".
This is the default and expected functionality, as it works the same way as regular module CSS overrides.

Attached is the patch that will scan whole theme directory when checking if the theme overrides the CSS.

Note: the same patch has already been posted for 1.x branch of the module.

Let me know if any questions - thanks!