qmk_firmware/keyboards/handwired/onekey/keymaps/rgb/keymap.c

#include QMK_KEYBOARD_H

const uint16_t PROGMEM keymaps[][MATRIX_ROWS][MATRIX_COLS] = {
    LAYOUT_ortho_1x1(UG_NEXT)
};

void keyboard_post_init_user(void) {
    rgblight_enable_noeeprom();
    rgblight_sethsv_noeeprom(HSV_CYAN);
    rgblight_mode_noeeprom(RGBLIGHT_MODE_RAINBOW_SWIRL);
}