:root {
  --clr-primary-strong-cyan: hsl(172, 67%, 45%);
  --clr-neutral-very-dark-cyan: hsl(183, 100%, 15%);
  --clr-neutral-dark-grayish-cyan: hsl(186, 14%, 43%);
  --clr-neutral-grayish-cyan: hsl(184, 14%, 56%);
  --clr-neutral-light-grayish-cyan: hsl(185, 41%, 84%);
  --clr-neutral-very-light-grayish-cyan: hsl(189, 41%, 97%);
  --clr-neutral-white: hsl(0, 0%, 100%);
  --clr-error: hsl(13, 71%, 61%);

  --ff-main: 'Space Mono', monospace;
  --fw-bold: 700;

  --fs-form-inputs: 1.5rem; /* 24px */
}

/* --- CSS Reset --- */
*,
*::before,
*::after {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}

body {
  font-family: var(--ff-main);
  background-color: var(--clr-neutral-light-grayish-cyan);
  display: flex;
  flex-direction: column;
  align-items: center;
  min-height: 100vh;
  padding: 2rem 0;
}

.logo {
  margin-bottom: 2.5rem;
}

.calculator {
  background-color: var(--clr-neutral-white);
  border-radius: 1.5rem;
  padding: 2rem;
  max-width: 920px;
  width: 100%;
  display: grid;
  gap: 2rem;
}

.calculator__inputs {
  display: flex;
  flex-direction: column;
  gap: 2rem;
}

.input-group label,
.tip-selection label {
  color: var(--clr-neutral-dark-grayish-cyan);
  font-weight: var(--fw-bold);
  display: block;
  margin-bottom: 0.5rem;
}

.input-wrapper {
  position: relative;
}

.input-icon {
  position: absolute;
  left: 1.2rem;
  top: 50%;
  transform: translateY(-50%);
}

.input-field {
  width: 100%;
  background-color: var(--clr-neutral-very-light-grayish-cyan);
  border: 2px solid transparent;
  border-radius: 0.3rem;
  padding: 0.5rem 1rem;
  font-family: var(--ff-main);
  font-size: var(--fs-form-inputs);
  font-weight: var(--fw-bold);
  color: var(--clr-neutral-very-dark-cyan);
  text-align: right;
  outline: none;
}

.input-field::placeholder {
  color: var(--clr-neutral-grayish-cyan);
}

.input-field:focus {
  border-color: var(--clr-primary-strong-cyan);
}

.input-field.error {
  border-color: var(--clr-error);
}

.input-group {
  position: relative;
}

.input-group label {
  display: flex;
  justify-content: space-between;
}

.error-message {
  color: var(--clr-error);
  display: none;
}

.tip-grid {
  display: grid;
  grid-template-columns: repeat(2, 1fr);
  gap: 1rem;
}

.tip-btn {
  background-color: var(--clr-neutral-very-dark-cyan);
  color: var(--clr-neutral-white);
  font-family: var(--ff-main);
  font-size: var(--fs-form-inputs);
  font-weight: var(--fw-bold);
  border: none;
  border-radius: 0.3rem;
  padding: 0.5rem;
  cursor: pointer;
  transition: background-color 0.2s, color 0.2s;
}

.tip-btn:hover {
  background-color: hsl(172, 67%, 75%);
  color: var(--clr-neutral-very-dark-cyan);
}

.tip-btn.active {
  background-color: var(--clr-primary-strong-cyan);
  color: var(--clr-neutral-very-dark-cyan);
}

.tip-custom {
  font-size: 1.25rem;
}

.calculator__results {
  background-color: var(--clr-neutral-very-dark-cyan);
  border-radius: 1rem;
  padding: 2.5rem 1.5rem 1.5rem;
  display: flex;
  flex-direction: column;
  gap: 1.5rem;
}

.result-group {
  display: flex;
  justify-content: space-between;
  align-items: center;
}

.result-label {
  color: var(--clr-neutral-white);
  font-weight: var(--fw-bold);
}

.result-label span {
  color: var(--clr-neutral-grayish-cyan);
  font-size: 0.8rem;
}

.result-amount {
  color: var(--clr-primary-strong-cyan);
  font-size: 2rem;
  font-weight: var(--fw-bold);
  line-height: 1;
}

.reset-btn {
  background-color: var(--clr-primary-strong-cyan);
  color: var(--clr-neutral-very-dark-cyan);
  font-family: var(--ff-main);
  font-size: 1.25rem;
  font-weight: var(--fw-bold);
  text-transform: uppercase;
  border: none;
  border-radius: 0.3rem;
  padding: 0.8rem;
  cursor: pointer;
  margin-top: auto;
  transition: background-color 0.2s;
}

.reset-btn:hover {
  background-color: hsl(172, 67%, 75%);
}

.reset-btn:disabled {
  background-color: hsl(183, 79%, 24%);
  color: hsl(183, 100%, 25%);
  cursor: not-allowed;
}

.attribution {
  font-size: 11px;
  text-align: center;
  margin-top: 2rem;
}
.attribution a {
  color: hsl(228, 45%, 44%);
}

@media (min-width: 900px) {
  body {
    justify-content: center;
  }
  .logo {
    margin-bottom: 5.5rem;
  }
  .calculator {
    grid-template-columns: 1fr 1fr;
    gap: 3rem;
    padding: 2.5rem;
  }
  .tip-grid {
    grid-template-columns: repeat(3, 1fr);
  }
  .calculator__results {
    padding: 2.5rem;
    gap: 2.5rem;
  }
  .result-amount {
    font-size: 3rem;
  }
}