* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}

:root {
  --color-white: #fff;
  --main-font: "Spartan", sans-serif;
}
.theme-1 {
  --main-bg: hsl(222, 26%, 31%);
  --keypad-bg: hsl(223, 31%, 20%);
  --screen-bg: hsl(224, 36%, 15%);

  --key-bg-1: hsl(30, 25%, 89%);
  --key-shadow-1: hsl(28, 16%, 65%);

  --key-bg-2: hsl(225, 21%, 49%);
  --key-shadow-2: hsl(224, 28%, 35%);

  --equals-bg: hsl(6, 63%, 50%);
  --equals-shadow: hsl(6, 70%, 34%);

  --key-color-primary: hsl(221, 14%, 31%);
  --key-color-secondary: hsl(0, 0%, 100%);
}
.theme-2 {
  --main-bg: hsl(0, 0%, 90%);
  --keypad-bg: hsl(0, 5%, 81%);
  --screen-bg: hsl(0, 0%, 93%);
  --key-bg-1: hsl(45, 7%, 89%);
  --key-shadow-1: hsl(35, 11%, 61%);
  --key-bg-2: hsl(185, 42%, 37%);
  --key-shadow-2: hsl(185, 58%, 25%);
  --equals-bg: hsl(25, 98%, 40%);
  --equals-shadow: hsl(25, 99%, 27%);
  --key-color-primary: hsl(60, 10%, 19%);
  --key-color-secondary: hsl(60, 10%, 19%);
}

.theme-3 {
  --main-bg: hsl(268, 75%, 9%);
  --keypad-bg: hsl(268, 71%, 12%);
  --screen-bg: hsl(268, 71%, 12%);
  --key-bg-1: hsl(268, 47%, 21%);
  --key-shadow-1: hsl(290, 70%, 36%);
  --key-bg-2: hsl(281, 89%, 26%);
  --key-shadow-2: hsl(285, 91%, 52%);
  --equals-bg: hsl(176, 100%, 44%);
  --equals-shadow: hsl(177, 92%, 70%);
  --key-color-primary: hsl(52, 100%, 62%);
  --key-color-secondary: hsl(52, 100%, 62%);
}

@media screen and (max-width: 720px) {
  html {
    font-size: 70%;
  }
}

body {
  height: 100vh;

  font-family: var(--main-font);
  background-color: var(--main-bg);

  display: flex;
  justify-content: center;
  align-items: center;
  transition: all 0.5s;
}

.app {
  font-size: 2rem;
}

.app__header {
  display: flex;
  justify-content: space-between;
  align-items: baseline;
  color: var(--key-color-secondary);
}

.app__logo {
  font-size: 1.5rem;
}

.app__toggle {
  display: flex;
  gap: 1rem;
  font-size: 0.7rem;
  align-items: flex-end;
}

.app__toggle-button {
  width: 4rem;
  display: grid;
  grid-template-columns: 1fr 1fr 1fr;
  justify-content: center;
  justify-items: center;
}
.app__toggle--toggle-area {
  cursor: pointer;
  width: 4rem;
  grid-column: 1/4;
  height: 1.2rem;
  border-radius: 2rem;
  background-color: var(--keypad-bg);
  grid-template-columns: 1fr 1fr 1fr;
  appearance: inherit;
  padding: 0 0.2rem;
}

.app__toggle--toggle-area::-webkit-slider-thumb {
  -webkit-appearance: none;
  background-color: var(--equals-bg);
  height: 0.8rem;
  border-radius: 50%;
  width: 0.8rem;
}

.app__screen {
  margin-top: 1.5rem;
  height: 6rem;
  background-color: var(--screen-bg);

  border-radius: 0.5rem;

  color: var(--key-color-secondary);

  display: flex;
  justify-content: flex-end;
  align-items: center;

  padding: 2rem;
}

.app__keypad {
  margin-top: 1.5rem;
  display: grid;
  grid-template-columns: repeat(4, minmax(3rem, 5rem));
  grid-template-rows: 1fr 1fr 1fr 1fr;
  gap: 1.5rem;
  padding: 2rem;
  background-color: var(--keypad-bg);
  border-radius: 0.5rem;
}

.app__keypad-btn {
  cursor: pointer;
  border-radius: 0.5rem;
  background-color: var(--key-bg-1);
  font-size: 1.5rem;
  font-family: inherit;
  color: var(--key-color-primary);

  padding: 0.6rem 0.5rem;

  border: none;
  box-shadow: 0 2px 0px 0px var(--key-shadow-1);

  transition: 0.15s transform;
}

.rst {
  grid-column: 1 / 3;
  color: var(--color-white);
}
.eq {
  grid-column: 3 / 5;
  background-color: var(--equals-bg);
  box-shadow: 0 2px 0px 0px var(--equals-shadow);
  color: var(--color-white);
}
.del,
.rst {
  background-color: var(--key-bg-2);
  color: var(--color-white);
  box-shadow: 0 2px 0px 0px var(--key-shadow-2);
}

.app__keypad-btn:hover {
  transform: translateY(-1px);
}
.app__keypad-btn:active {
  transform: translateY(0.1rem);
}