If you travel to big cities in summer and look civilized and not an ignorant cartoon tourist, then the default would be
- jeans shorts
- a cap or a hipsty hat
The biggest difficulty in big cities in summer is heat. Temperature is usually around 26..32°C (especially on the sunny side of the street), and air conditioning is not as available as in tropical countries. So usual trouses and long-sleeve shirts make you sweat quickly and profusely.
Shorts are fine down to +20°C if it's not very windy. If you've been caught dressed this lightly at night then stick to staying indoors (although in summer temperatures rarely drop below 17°C at night, so no frostbite). Many kerbside cafes offer warm blankets.
But beware: if you board a ship for an excursion, clothes must be heavier. Air above water is colder and it's always windy.
For colder summer weather, a long-sleeve shirt and usual trouses/jeans are enough. For rare cold weathers in early or very late summer, you may carry a light jacket in the suitcase. That's all you need.
You may buy those items mentioned above at Tommy Hilfiger if you want to look rich, or at less expensive stores, and you'll look perfectly like an average (and trendy) big city person.
I can't imagine a clothes style that could be really out of place in big city. Overdoing with bright and too pretentious clothes may seem weird. Things only change in small towns where self-expression is still not a value, and cheap sportswear is quite dominant among less educated people. But jeans and t-shirts are the default clothes for any place from subtropics to arctic (not at ceremonies though).
I don't recall any troubles with Churches, they might only ask men to take off hats.