Twitter today is opening up applications for its new testing program, first announced at CES in January. The program potentially can cover any and every aspect of the Twitter experience, but the first set of tests will focus on how interactions between people, and specifically replies, appear on Twitter. They will include a new design for replies to make it easier to follow a conversation; rounded shapes on reply tweets; indents to follow responses; hiding engagement and sharing behind a tap to bring out the content of replies; and introducing colors to add more context. We got an early look at the these features when we saw the beta app in January; read on for more on all the features below. Improving the look and feel of how Twitter works is a tall order, to say the least.