FIG. 1 is a front left perspective view illustrating our new design for a dimmer switch with the broken lines showing, for illustrative purposes only, a back plate and a housing that are not part of the claimed design;

FIG. 2 is a front view thereof with the dashed lines showing the back plate;

FIG. 3 is a left side view thereof with the broken lines showing the back plate and the housing;

FIG. 4 is a right side view thereof with the broken lines showing the back plate and the housing;

FIG. 5 is a top view thereof with the broken lines showing the back plate and the housing; and,

FIG. 6 is a bottom view thereof with the broken lines showing the back plate and the housing.