The ornamental design for the lighting fixture backplate, as shown and described.
FIG. 1 is a right side elevational view of the lighting fixture backplate
in accordance with my design illustrated in the environment of a lamp, the
left side being a mirror image thereof;
FIG. 2 is a top plan view of the lighting fixture backplate in accordance
with my design, the bottom being identical thereto;
FIG. 3 is a rear side elevational view of the lighting fixture backplate in
accordance with my design; and,
FIG. 4 is a front side elevational view of the lighting fixture backplate
in accordance with my design.
The broken lines show the environment in which the article is used and are
not considered part of the design claimed.