  We consider scalar-vector-tensor (SVT) theories with second-order equations
of motion and tensor propagation speed equivalent to the speed of light. Under
the sub-horizon and the quasi-static approximations we find analytical formulae
for an effective dark energy fluid, i.e., sound speed, anisotropic stress as
well as energy density and pressure. We took advantage of our general,
analytical fluid description and showed that it is possible to design SVT
cosmological models which are degenerate with $\Lambda$CDM at the background
level while having gravity strength $G_{\rm eff}<G_{\rm N}$ at late-times as
well as non-vanishing dark energy perturbations. We implemented SVT designer
models in the widely used Boltzmann solver CLASS thus making it possible to
test SVT models against astrophysical observations. Our effective fluid
approach to SVT models reveals non trivial behaviour in the sound speed and the
anisotropic stress well worth an investigation in light of current
discrepancies in cosmological parameters such as $H_0$ and $\sigma_8$.
