The new theory at Theory, a brand best known for stylish, workhorse professional clothes, is to put fashion-world darling Olivier Theyskens in charge of artistic direction.

The company announced the appointment of the former Rochas and Nina Ricci designer to the top design spot on Monday.

Theyskens had already started a capsule collection as a brand collaborator for the upcoming spring season, but this deal puts him in charge of all Theory collections starting with fall 2011.

“It is really exciting to be able to offer my point of view as a designer to a brand that holds a leading role in contemporary fashion. Working on all collections of Theory as well as Theyskens’ Theory will enable me to explore various design directions and express my creativity in a new and broader way,” Theyskens said in a statement.

Theory founder and CEO Andrew Rosen said he finds the work of the Belgian-born, 33-year-old to be “inspiring.”

Theyskens won the Council of Fashion Designers of America award for international designer of the year in 2006, and earlier this year worked with publisher Assouline and photographer Julien Claessens on “The Other Side of the Picture,” a coffee-table book that showed off fashion shows from the point of view of an artistic insider.

“His energy and creativity will provide a new spark for the organization, and for the whole Contemporary marketplace,” Rosen said.