So, Showa Year 90 is basically saying "2015," except the Showa Period (In Japan, there is a way of numbering years in accordance with the number of years a particular emperor has been in rule, I'm ashamed that I'm not more well-versed on the matter than that. (background on Showa Period
here))
I haven't listened, but it sounds to me like it may have been made with the concept of "what if the present day linearly followed that time?" And given that it's Urbangarde, the lyrical and visual content probably deals with concepts like nostalgia for simpler times, love and despair, youth, freedom, violence/death, purity/sex, likely with visual or conceptual references to war, apocalypse, suicide, and/or imperialism. These seem to be ideas that they include in all of their work. The singles and videos leading up to it were pretty good.