We claim the ornamental design for a light string power adapter, as shown and described.
FIG. 1 is a perspective view of an ornamental design of a light string power adapter;
FIG. 2 is a left-side view of the light string power adapter of FIG. 1;
FIG. 3 is a top view of the light string power adapter of FIG. 1;
FIG. 4 is a right-side view of the light string power adapter of FIG. 1;
FIG. 5 is a bottom view of the light string power adapter of FIG. 1;
FIG. 6 is a front view of the light string power adapter of FIG. 1; and,
FIG. 7 is a rear view of the light string power adapter of FIG. 1.
The figures include broken lines that are directed to environment and are for illustrative purposes only; the broken lines form no part of the claimed design.