It's not too late! You can still add under-cabinet lighting solutions. Just look for surface mount fixtures since your cabinets are already installed.

Some advice on under-cabinet lighting is here: http://www.lightsonline.com/kitchen-under-cabinet-lights

Pay attention to the bit about where to mount the lights so as to prevent glare. Since onyx is a dark material, it absorbs more light, so make sure your under-cabinet lights have a high lumen output to prevent the light from getting lost in the shuffle. You may also wish to consider lights with lenses over the light bulbs, which helps prevent reflective glare bombs.

We have a wide selection of under-cabinet products: http://www.lightsonline.com/under-cabinet-lights/

Hope this helps! Let us know if you need more advice or product recommendations.

Photos?

Lights Online - the OP said 'under counter', not cabinet.

Whoops! Well, actually, a lot of the same considerations and materials are used here--under-cabinet lights can certainly be mounted underneath countertops--in fact, we just wrote about this very topic with Houzz examples: http://bit.ly/TK1UFR