so... i have a blackwidow 2014 that dont have leds, but i can see the configuration to do a solder on , i dont have a multimeter and leds to test, i want to know if i can solder any led i want.
If the keyboard did not come with keycap LEDs,
Soldiering a key switch with a built in LED is not going to light up, because the chip board does not come with key switch LED configuration.
You can try use the following youtube videos as a guide to solder on the LEDs, but I don't think it will work on your keyboard