There are several companies which have focused on WordPress customization services and are specialized to make small design changes to existing themes, add new features or change nearly everything on the theme you want.

Werkpress
werkpress.com

Codeable
codeable.io

WooNinjas
wooninjas.com

WPCurve
wpcurve.com

You can also hire a WordPress freelancer on these platforms:

• WordPress Jobs
• WP Hired
• Smashing Jobs
• Freelancer.com
• FreelanceSwitch