I claim the ornamental design for the toilet seat and base cover, as shown and described.
FIG. 1 is a top view of the toilet seat and base cover.
FIG. 2 is a bottom view of the claimed design.
FIG. 3 is a left side view of the claimed design.
FIG. 4 is a rear view of the claimed design.
FIG. 5 is a front view of the claimed design; and,
FIG. 6 is a perspective view of the claimed design in use. The toilet seat, toilet base, and toilet tank depicted in broken lines in FIG. 6 is environmental matter, and forms no part of the claimed design.