Under Setup > General > Look & Feel, you can change the settings to 'Custom' and enter your own hex/color values. Refresh Preview to see how it looks.
There are some CSS style sheets you can grab, from the same page, as well.
Were you able to figure this out? I just upgraded last night and can't figure out how to change the head/title bar area to color other than black.
If you are on about the login screen or client interface, changing the Look & Feel with Custom CSS will achieve this.
If you are on about the dark headers and title bars within the technician interface, this is not possible by any default configuration or option available within the help desk. If required, you would need to change the backend files so all CSS values would change the overall look.
Please note that going in backend and changing these values is unsupported and that if achieved, any update or hotfix applied will likely reset any configuration changes you make.