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

FIG. 2 is a top view thereof with the broken lines showing, for illustrative purposes only, the back plate and housing which are not part of the claimed design;

FIG. 3 is a front view thereof with the dashed lines showing the back plate;

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

FIG. 5 is a left side elevational 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.