I found out last night that my PC doesn't turn it's USB off when it's shut down, so I've added a switch and a capacitor (for a fade effect), there is a circuit diagram on the making stage.
Step 1: Collect Your Parts! (..and Tools)
2x 3mm Blue LEDs (High brightness or normal, it's up to you)
1x 56ohm resistor
A male USB connetion
a length of cable.
Now, this is better if you have a PC, laptop or anything with a USB connection nearby, but you could do my inctructable on making a battery operated 5v power supply (https://www.instructables.com/id/Simple-5v-PSU-battery-powered/) and then combine it with this to make your undershelf lighting.
Step 2: Make It
Step 3: Fit It.
Step 4: Admire.
Looks great doesn't it?