The ornamental design for a theaterstage, as shown and described.
FIG. 1 is a first perspective view of a theater stage showing my design;
FIG. 2 is a top view thereof;
FIG. 3 is a top view thereof, shown without stippling, including reference to a front cross-sectional view;
FIG. 4 is a front cross-sectional view taken from 4-4 of FIG. 3;
FIG. 5 is a top view thereof, shown without stippling, including reference to rear cross-sectional view;
FIG. 6 is a rear cross-sectional view taken from 6-6 of FIG. 5; and,
FIG. 7 is a second perspective view taken from the reverse view of FIG. 1.
The broken lines showing of details in FIGS. 1-7, including the stairs under the stage and the outer sides and bottom walls of the stage is included for the purpose of showing environmental portions of the theater stage and forms no part of the claimed design.