The ornamental design for the thumb brace, as shown and described.
FIG. 1 is a right perspective view of the thumb brace where a wearer's hand is in broken lines to show the environment of the invention, but form no part of it;
FIG. 2 is a bottom plan view of the thumb brace with twice folded straps;
FIG. 3 is a left front perspective view of the thumb brace shown in FIG. 2;
FIG. 4 is a left front perspective view of the thumb brace with folded straps;
FIG. 5 is a left front perspective view of the thumb brace with extended straps;
FIG. 6 is a back plan view of the thumb brace shown in FIG. 5; and,
FIG. 7 is a back plan view of the thumb brace shown in FIG. 4.
The broken lines in FIG. 1 through FIG. 7 are included for the purpose of illustrating portions of the thumb brace that form no part of the claimed design.