app.css

button {
  border: 0;
  font-weight: 500;
}

fieldset {
  margin: 0;
  padding: 0;
  border: 0;
}

#form-container {
  position: relative;
  width: 380px;
  margin: 0 auto;
  top: 50%;
}

.third {
  float: left;
  width: calc((100% - 32px) / 3);
  padding: 0;
  margin: 0 16px 16px 0;
}

.third:last-of-type {
  margin-right: 0;
}

/* Define how SqPaymentForm iframes should look */
.sq-input {
  height: 56px;
  box-sizing: border-box;
  border: 1px solid #E0E2E3;
  background-color: white;
  border-radius: 6px;
  display: inline-block;
  -webkit-transition: border-color .2s ease-in-out;
  -moz-transition: border-color .2s ease-in-out;
  -ms-transition: border-color .2s ease-in-out;
  transition: border-color .2s ease-in-out;
}

/* Define how SqPaymentForm iframes should look when they have focus */
.sq-input--focus {
  border: 1px solid #4A90E2;
}

/* Define how SqPaymentForm iframes should look when they contain invalid values */
.sq-input--error {
  border: 1px solid #E02F2F;
}

#sq-card-number {
  margin-bottom: 16px;
}

/* Customize the "Pay with Credit Card" button */
.button-credit-card {
  width: 100%;
  height: 56px;
  margin-top: 10px;
  background: #4A90E2;
  border-radius: 6px;
  cursor: pointer;
  display: block;
  color: #FFFFFF;
  font-size: 16px;
  line-height: 24px;
  font-weight: 700;
  letter-spacing: 0;
  text-align: center;
  -webkit-transition: background .2s ease-in-out;
  -moz-transition: background .2s ease-in-out;
  -ms-transition: background .2s ease-in-out;
  transition: background .2s ease-in-out;
}

.button-credit-card:hover {
  background-color: #4281CB;
}