  Both theoretical interest and practical significance attach to the sign and
strength of Casimir forces. A famous, discouraging no-go theorem states that
"The Casimir force between two bodies with reflection symmetry is always
attractive." Here we identify a loophole in the reasoning, and propose a
universal way to realize repulsive Casimir forces. We show that the sign and
strength of Casimir forces can be adjusted by inserting optically active or
gyrotropic media between bodies, and modulated by external fields.
