The ornamental design for a shower mixer trim, as shown and described.
FIG. 1 is a front perspective view of a shower mixer trim according to the present invention.
FIG. 2 is a front view of the shower mixer trim of FIG. 1.
FIG. 3 is a rear view of the shower mixer trim of FIG. 1.
FIG. 4 is a left side view of the shower mixer trim of FIG. 1.
FIG. 5 is a right side view of the shower mixer trim of FIG. 1.
FIG. 6 is a top view of the shower mixer trim of FIG. 1; and,
FIG. 7 is a bottom view of the shower mixer trim of FIG. 1.
The dashed lines shown on the top surfaces of the knobs and on the rear of the shower mixer trim define the bounds of the claimed design and no claim is made to the material illustrated within the dashed lines thereof.