You're missing a projection.

([point to rotate] - [center of rotation]) * [rotation matrix] + [point to rotate]

Writing it explicitly for some point, it should look like:

x' = (x - x_c) * cos(theta) - (y - y_c) * sin(theta) + x_c

y' = (y - y_c) * cos(theta) + (x - x_c) * sin(theta) + y_c