Customizing Ploopy firmware with VIA

Beginner question about QMK firmware:

I've just bought and assembled a Ploopy Classic 2, and it came with VIA firmware pre-loaded. I want to change the behavior of the DPI_CONFIG macro so it cycles between 3 options instead of 5. I cloned the qmk_firmware repo, modified PLOOPY_DPI_OPTIONS, and flashed the firmware. But I lost all VIA functionality. So I went back to the default firmware from https://caniusevia.com. How can I customize this feature while preserving VIA functionality?