Theo played the 1998-1999 season with the Colorado Avalanche, was acquired as a free agent by the New York Rangers in 1999, and then played for the Chicago Blackhawks in 2002. In 2002 he won an Olympic gold medal as a member of the Canadian hockey team.
Theo is known for his fiestiness on the ice, and his determination. Diagnosed with Crohn's disease in 1996, he has been determined not to let the disease rule his life.
Unfortunately, on April 11, 2003, Theo was suspended indefinitely by NHL for violating the terms of his substance abuse aftercare program. He played the 2005-06 season with the Belfast Giants of the British Elite Ice Hockey League, and was named 2006 Kingdom of the Giants Most Valuable Player, but decided not return for the 2006-07 season.
In 2009, the NHL lifted the suspension and Theo returned to the NHL, playing some preseason games with the Calgary Flames. After some soul-searching, the 41-year-old decided not to continue playing and instead decided to retire.
Theo has written an autobiography with Kirstie McLellan Day entitled Playing With Fire, which was published in October 2009.