In this first paper we discuss the linear theory and the background evolution of a new class of models we dub SCDEW: Strongly Coupled DE, plus WDM. In these models, WDM dominates today's matter density; like baryons, WDM is uncoupled. Dark Energy is a scalar field Φ; its coupling to ancillary CDM, whose today's density is ≪ 1 %, is an essential model feature. Such coupling, in fact, allows the formation of cosmic structures, in spite of very low WDM particle masses (∼ 100 eV). SCDEW models yields Cosmic Microwave Background and linear Large Scale features substantially undistinguishable from ΛCDM, but thanks to the very low WDM masses they strongly alleviate ΛCDM issues on small scales, as confirmed via numerical simulations in the II associated paper. Moreover SCDEW cosmologies significantly ease the coincidence and fine tuning problems of ΛCDM and, by using a field theory approach, we also outline possible links with inflationary models. We also discuss a possible fading of the coupling at low redshifts which prevents non linearities on the CDM component to cause computational problems. The (possible) low-z coupling suppression, its mechanism, and its consequences are however still open questions -not necessarily problems-for SCDEW models. The coupling intensity and the WDM particle mass, although being extra parameters in respect to ΛCDM , are found to be substantially constrained a priori so that, if SCDEW is the underlying cosmology, we expect most data to fit also ΛCDM predictions.