Dimming USB Led Lamp
Dimming LEDs is usually done with PWM which means simply switching them on and off fast enough. If I understood correctly, you have dumb lamps connected to USB draining power directly from USB. You can not easily switch USB power on and off fast enough so you will have to plug some device between your lamp and the USB port. If you want to control it from the Raspberry Pi, you would need a circuit around some IC, a USB device driver in Linux and even more.

I guess it would be easier to use the GPIO pins of the Raspberry Pi. Those should be programmable rather easily and you would need just a very simple circuit and a software PWM for the GPIO.