Margravial Opera House in Germany
The Margravial Opera House in Bayreuth, Germany, was built in 1748 and became a UNESCO World Heritage Site in 2012. "A masterpiece of Baroque theatre architecture, ... the Opera House is the only entirely preserved example of its type where an audience of 500 can experience Baroque court opera culture and acoustics authentically, as its auditorium retains its original materials, i.e. wood and canvas," the UNESCO website says. The wooden interior's unique streaked marble and carved, painted decorations were designed by Italian architect Guiseppe Galli Bibiena.
Unfortunately, visitors cannot currently witness this box theater's red, blue and gold majesty, as it's closed for renovation until 2018.