If you're hunting for a relaxed casual but also sharp look, try pairing a white and blue crew-neck t-shirt with blue chinos. You know how to spruce up this look: black leather derby shoes. As a 30-something Millennial gent, you probably want to start dressing maturely. In this case, looks like this are perfect as inspiration.
A white and blue crew-neck t-shirt and blue chinos? This is easily a wearable outfit that you can wear on a daily basis. Casual combinations for 30-something guys aren't actually that hard, as you see here.
Wear a white and blue crew-neck t-shirt with blue chinos for a practical getup that's also put together. This ensemble proves that in your thirties you have a wide array of dressing options.