style.css

html {
  box-sizing: border-box;
  font-size: 10px;
  background: #8E24AA;
  background: linear-gradient(45deg,  #42a5f5 0%,#478ed1 50%,#0d47a1 100%);
}

*, *:before, *:after {
  box-sizing: inherit;
}

body {
  margin:0;
  text-align: center;
  font-family: 'Inconsolata', monospace;
}

.display__time-left {
  font-weight: 100;
  font-size: 20rem;
  margin: 0;
  color:white;
  text-shadow:4px 4px 0 rgba(0,0,0,0.05);
}

.timer {
  display:flex;
  min-height: 100vh;
  flex-direction:column;
}

.timer__controls {
  display: flex;
}

.timer__controls > * {
  flex:1;
}

.timer__controls form {
  flex:1;
  display:flex;
}

.timer__controls input {
  flex:1;
  border:0;
  padding:2rem;
}

.timer__button {
  background:none;
  border:0;
  cursor: pointer;
  color:white;
  font-size: 2rem;
  text-transform: uppercase;
  background:rgba(0,0,0,0.1);
  border-bottom:3px solid rgba(0,0,0,0.2);
  border-right:1px solid rgba(0,0,0,0.2);
  padding:1rem;
  font-family: 'Inconsolata', monospace;
}

.timer__button:hover,
.timer__button:focus {
  background:rgba(0,0,0,0.2);
  outline:0;
}

.display {
  flex:1;
  display:flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
}

.display__end-time {
  font-size: 4rem;
  color:white;
}