When 'severe' is used to describe clothing styles it means the clothes are very fitted and with no unnecessary decoration and usually very limited in colour variation. A well fitted, simple black suit, white shirt, black tie and plain black shoes could be regarded as severe.

Dark trousers , shirt and tie are several kinds/style of clothing. Those 3 items are types of clothing . More than one item means it must take the plural form , as in , "are" Clothing is what your wear, like sweaters, shirts, pants, dresses, and so on. Your favourite piece of clothing is.... Trousers ARE a piece of clothing > Trousers ALWAY take the plural form , just like pants , glasses, scissors or many other paired items. So. trousers is .,, > incorrect If you want to take the singular form , then say, a pair of trousers is hardly enough for you .