I do not believe it would be a grave sin to pray for a team to win,but God would not answer that prayer.
A whole team could confess Christ,and pray every game to win,and go to Church,and feed the poor and hungry,and God would still not answer that prayer for that team to win.
God is not concerned with the endeavors of the world that would have to do with people spending money to watch the sport,and anything associated with something that has people spending money on something that is not a necessity,and all that money spent on a lust of the flesh for entertainment which neglects the poor and needy which is more important.
Not saying that God does not love the sports player that confesses Christ,and not working in his life,but according to his profession of who wins and loses,it is a determination based on the players abilities alone.
If a person is involved in a job that provides something to the public that has to do with their needs,then I can see God blessing that person,and that company.
Also sports stars are known to live lavishly,and in great measure,and have we seen one player who only goes by their needs,and not their wants,and distributes their money to poor,which Paul said having food and clothing be content,and the saints appear poor to the world,but they possess all things to be given them in the future.
Sports is not a necessity,and we do not need it to survive,so prayer would not be effective.
Also for God to bless one team He would have to curse another which He will not do.
Go ahead watch the game,hope,wish,desire,dream,anticipate,yearn,crave,anticipate,optimistic,thirst,and want your team to win.
But do not pray for your team to win,and please do not go to the desperate attempt of praying to Allah,or Guanyin,or Ahura Mazd,or Queztalcoatl,or Biame,or Jupiter Optimus Maximus,or Zeus,or Breged,the Mother Goddess,or Ra,or Vishnu,or the Great White Spirit in the sky,because you want your team to win.
You can gain all the winning sports teams in the world but what does it profit if you lose your soul,lol.