Hi Andreas,
Please email me at andrew@writeonweb.com - I can help you integrate a custom CSS design for products on sale. Please email any designs you may have already, or thoughts if design is still pending (even references to other sites is helpful).
I'm a Shopify Partner, with extensive experience in the Shopify platform along with HTML5, JavaScript, and CSS3 cross device and browser display issues. I can navigate any custom development code relatively quickly, and help you in the long term as well beyond display issues including data, shopify template logic, JavaScript, etc.
Best!
~Andrew (Seattle based :-)