Austere

1. Grim or severe in manner or appearance

2. Lacking trivial decoration; not extravagant or gaudy
3. The interior of the church was as austere as the parishioners were dour.

(grim or severe): stern, strict, forbidding

(lacking trivial decoration): simple, plain, unadorned, unembellished

(not lacking trivial decoration): overwrought, flamboyant, extravagant, gaudy, flashy