Need help modifying the New Order Received email in Woocommerce

I’ve looked around, and not been able to find the info I need.
I want to change the default email layout significantly
Using help from a nice infographic on tyche softwares website (link) I have dispensed with the footer area, and the background, etc.
What I am doing is an online order system for a cafe. When the client purchases their order, the order confirmation email goes to the Chef’s printer and spits out an order slip.
I want the order number BIG and LOUD and all that. And I’ve had partial success: this is what I’ve achieved so far.

I want to make the background (which should be in admin-new-order.php and email-header.php) Black and full width of the cell. I also want to greatly increase the font size. But I’ve tried editing the two files, and cannot seem to get this ‘background’ to full width, nor the font size to say 6em or similar

I’m bound to be making a simple mistake or just fudging php and html etc. Any suggestions?