Etienne is 100% a bloke's name. It is NOT unisex in any way. It is NOT used as a tom-boyish name for a girl. Using this name for a girl would be an awful, humiliating blunder. It's like calling a girl Edward, Richard, Derek, Brian, William. It would just demonstrate that the parents had no idea what they were doing and had been too stupid as to look into the name.
It's not just a matter of a couple of raised eyebrows when the girl is on holidays in France. Every French speaker, and many, many others, she encounters in the UK and anywhere she goes in the world will think "oh.my.god, this poor girl, her parents obviously had no idea when they named her what they were naming her; she/they must have regretted it inumberabke times ever since".
It is on a par with the worst name blunder I ever came across. A very, young French woman in France, who I met, who named her son Wendy because she thought it was a great sounding English name. After years of embarassment, the poor lad now calls himself Vendy.
Re. the connection with football, St. Étienne is the main football team in Lyons. It has a massive following.
Luckily, there are many, many names that can yield the nickname, Etty. I'm sure you'll have no trouble at all finding one that you love that isn't a (100%) male name.