In the case of buildings used by or designed for multiple tenants, all signs shall express a single design theme, utilizing uniform size, color, materials, lighting and styles. In the case of existing buildings, new signs shall conform to the existing design theme, if a theme exists. In the case of existing buildings with more than one design theme, new signs shall conform to the single design theme which is expressed by the greatest number of existing signs. In the case of existing buildings where no design theme is expressed by more than one sign, new signs shall conform to the design theme expressed by any single existing sign. The above regulations shall not be construed to permit any sign which does not conform with all the requirements of this chapter.