We claim the ornamental design for a dimmer switch, as shown and described.
FIG. 1 is a perspective view of a dimmer switch according to our new design.
FIG. 2 is a front view thereof.
FIG. 3 is a left side view thereof.
FIG. 4 is a right side view thereof.
FIG. 5 is a top view thereof; and,
FIG. 6 is a bottom view thereof.
The rear view forms no part of the design and is omitted. The portions of the drawings appearing in broken line are for environment only and do not form a part of the design.