My mom's toenails have surface & have darker color than usual. What could be the reason & treatment? Thank you.

Other possibilities. In addition to the common causes enumerated by dr. Kopes-kerr, there are several distinctive patterns of nail discoloration which signal underlying medical conditions or syndromes. A nail expert (usually a dermatologist) should examine your mother. Btw greenish discoloration is often caused by pseudomonas bacteria, not fungus.
Nail coloration. Common causes of nail discoloration include bleeding under the nail (from bumping the toe) or chronic fungal infection, which both thickens and discolors the nail. With fungus infection it usually darkens by becoming opaque and whitish first; it bruises more easily and become dark from blood beneath the nail, and over time may develop yellowish or greenish discoloration from the fungus.