The "You Won!" text isn't handled by the PetBattleUI; it's handled by the LevelUpDisplay. Specifically:
Code:
LevelUpDisplay (frame)
↳ LevelUpDisplayLevelFrame (frame)
↳ LevelUpDisplayLevelFrameTextLine (fontstring)
See also: FrameXML\LevelUpDisplay.lua and FrameXML\LevelUpDisplay.xml.