30 degrees Fahrenheit is below freezing.
At 30 degrees Celsius, most people turn on their air conditioners.
We travel to other pcaels and into other cultures to learn, be amazed and to understand people who are different from ourselves. We don't go there to make fashion statements. As guests in other peoples' countries, our clothes should be neutral. We are visiting to be impressed, not to impress. That is not to say you can't wear clothes that are in style. But as you will certainly want to do lots of exploring, as Prague is a beautiful city filled with exciting things to see and do, you will also want to be comfortable. March will indeed be a bit chilly, and with all the walking you will want to do, you will want comfortable, sensible footwear and suitable outerwear. For an idea of what people are wearing, Google Webcams for Prague and check out the peoples' fashions. However, many Web cams primarily cover sites popular with visitors, so the people you observe may not be exactly representative of what people who actually live and work in Prague are wearing. In short, don't worry so much about clothes and buying a whole new wardrobe. Start researching all the wonderful things to see and enjoy in Prague. Have a great trip!