The original controller get's the charge status (charging or not charging) via a pin and accordingly makes an LED blink. The substitution for that is a wire bridging the ingoing and outgoing ping, resulting in the charge status LED glowing when the cat charges.

Because the original controller has no IC markings and I couldn't determine the type, I decided to desolder it and replace it with an Attiny85. I soldered the DIP-8 version of this controller onto a double sided prototyping board and filled and wired the holes in the middle in a way that I can program it with the use of pogo pins. Then I hot glued it onto a place on the PCB where the LEDs wouldn't cast a shadow.

The last step was to wire the power traces to the power pins, the RGBW traces to PB0..3 and the sensor trace to PB4.