This isn't my style.
'Stile' — 'STI-le'. Two syllables, stress on first. The 'e' at the end is not silent in Italian.
A polite but clear way to decline an item that doesn't match your aesthetic. Italians respect personal style and won't push back on this.
'Il mio stile' = my style. 'Non è il mio stile' is a complete, simple negation. 'Questo' refers to the item tried on. This phrase closes the door on a particular style without insult.
Non fa per me.
It's not for me.
More idiomatic — 'fare per qualcuno' = to be right for someone
Non mi appartiene come stile.
It doesn't belong to me as a style.
More philosophical — style as personal identity
Preferisco qualcosa di più sobrio.
I prefer something more understated.
Giving direction toward what you do want
Italy has several distinct regional style identities. Milanese style is minimalist and structured; Roman style is more theatrical; Neapolitan is colourful and expressive. Knowing your Italian style tribe helps communicate your preferences.