  The conventional Skyrme interaction is generalized by adding zero-range
charge-symmetry-breaking and charge-independence-breaking terms, and the
corresponding energy density functional is derived. It is shown that the
extended model accounts for experimental values of mirror and triplet
displacement energies (MDEs and TDEs) in sd-shell isospin triplets with, on
average, about 100~keV precision using only two additional adjustable coupling
constants. Moreover, the model is able to reproduce, for the first time, the
A=4n versus A=4n+2 staggering of the TDEs.
