The most likely scenario is: The proportion of white-furred rabbits in the population declines.
As the average temperature increases and winters are shorter, the environment may favor the brown-furred rabbits over the white-furred rabbits. White fur provides camouflage in snowy conditions, so with less snow and shorter winters, brown-furred rabbits would likely have better survival and reproductive success, leading to a decline in the proportion of white-furred rabbits.