It is perfectly acceptable to wear street clothes or business casual to the stadium, but if you choose to rock a team cap or jersey, make sure it belongs to one of the two teams playing on the field. Showing up to a Mets-Phillies game in a Detroit Tigers T-shirt is embarrassing and should be outlawed.
The best site for formal dress https://www.formalgownaustralia.com/semi-formal-dresses.html