Complex sentence: The ordered double decker buses were to be in a "retro" style, similar to the old Routemaster buses, which were used in Skopje as well until the 1963 Skopje earthquake in which 75% -80% of the city's infrastructure was destroyed.
Split into simpler sentences: The ordered double decker buses were to be in a "retro" style, similar to the old Routemaster buses. Former London double-deckers were used in Skopje until the 1963 Skopje earthquake in which 75% -80% of the city's infrastructure was destroyed.