What's the simplest (smallest) way to dim a DC LED strip with a switch?
Most efficient way is to use a circuit that pulses the 12V at high enough frequency to not be seen as flickering, then determine experimentally to find the duty cycle that yields perceived 50% dimness.

Basically a 555 set up as an oscillator an a N-channel mosfet between the cathodes and GND would do it.
Or an Arduino Promini and AOD508 (or equivalent Low Rds part) if you want to control the duty cycle in software instead of using Rs &amp; Cs.

Since these appear to be "dumb" (vs individually controllable chip) light strips, 4 transistors are needed, one for each color. Some mixing of colors may be needed to achieve the color and dimness you wish.  In which case, a Promini + 4 transistor approach would be smarter.  If you look the at controller at this site, something similar would be found.
https://www.superbrightleds.com/moreinfo/clearance-led-strip-and-tape-lights/outdoor-rgbw-led-strip-lights-weatherproof-12v-led-tape-light-w-white-and-multicolor-leds-245-lumensft/1711/#tab/specifications