style.css

body {
  background: #F8F9FA;
}

.target-time {
  width: 65px;
  margin-right: 10px;
  text-align: center;
  background: none;
  border: none;
  outline: none;
  font-size: 2em;
}

.time-cnt {
  display: flex;
  justify-content: center;
  height: 40px;
}

.time-info {
  align-self: flex-end;
  margin-right: 20px;
  font-size: 1.5em;
  transform: translate(-12px, -3px);
}

.seconds {
  margin: 0;
}

.title {
  text-align: center;
  margin-top: 50px;
}

.calc-container {
  display: flex;
  flex-direction: column;
  justify-content: center;
  align-items: center;
  margin-top: 80px;
  width: 500px;
  margin: auto;
}

h3 {
  margin-top: 30px;
}

button {
  margin-top: 30px;
}

input[type=number]::-webkit-inner-spin-button, 
input[type=number]::-webkit-outer-spin-button {  
   opacity: 1;
}

.goal-pace {
  display: flex;
  justify-content: space-evenly;
  margin-top: 50px
}

.goal-pace h4 {
  font-size: 55px;
}

@media (max-width: 1100px) {
  .goal-pace {
    flex-direction: column;
    justify-content: center;
    align-items: center;
  }
}

@media (max-width: 520px) {
  .goal-pace h4 {
    font-size: 30px;
  }

  .calc-container {
    width: 330px;
  }
}