The ornamental design for the lighting fixture backplate, as shown and described.
FIG. 1 is a side elevation view of the lighting fixture backplate in accordance with my design, illustrating a possible environment for the backplate;
FIG. 2 is a left side elevation view of the lighting fixture backplate in accordance with my design;
FIG. 3 is a front elevation view of the lighting fixture backplate in accordance with my design; and,
FIG. 4 is a bottom plan view of the lighting fixture backplate in accordance with my design.
The broken lines showing environmental structure are for illustrative purposes only and form no part of the claimed design. The rear elevation view is unornamented and follows the contours of FIGS. 2 through 4 and is not part of the claimed design. The right and left views are mirror images. The top and bottom views are mirror images.