Theodore, who signed a two-year, $3 million contract with Florida during the off-season, faced 22 shots over the final two periods but was up the task, handing the Panthers a victory in their season opener.
Florida Coach Kevin Dineen was rewarded with a win in his first game behind the Panthers' bench.
First-period goals from Stephen Weiss and Jason Garrison provided Florida's offense.
Al Montoya stopped 27-of-29 shots for the Islanders.
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