style.css

@import url('https://fonts.googleapis.com/css2?family=Montserrat&display=swap');

:root {
  --color: #090909;
  --background-color: #efefef;
  --main-calculator-color: #e8e8e8;
  --calculator-hover-border-color: #ffffff;
  --calculator-box-shadow: 20px 20px 60px #c5c5c5, -20px -20px 60px #ffffff;
  --calculator-display-box-shadow: inset 5px 5px 15px #cecece,
    inset -5px -5px 15px #ffffff;
  --calculator-button-box-shadow: 6px 6px 14px #c7c7c7, -6px -6px 14px #f9f9f9;
  --calculator-button-box-shadow-active: 4px 4px 14px #c7c7c7,
    -4px -4px 14px #f9f9f9;
  --toggle-theme-switch-width: 62px;
  --toggle-theme-switch-height: 29px;
  --toggle-theme-switch-icon-size: 16px;
  --toggle-theme-switch-icon-background: linear-gradient(
    145deg,
    #d9d9d9,
    #f8f8f8
  );
  --toggle-theme-switch-color: #efefef;
  --toggle-theme-switch-box-shadow: 3px 3px 6px #c7c7c7, -3px -3px 6px #f9f9f9;
  --toggle-theme-switch-slider-box-shadow: 1px 1px 2px #bcbcbc,
    -1px -1px 2px #ffffff;
  --footer-color: #000000;
  --footer-link-color: #656565;
}

body.dark-theme {
  --color: #aeaeae;
  --background-color: #1e1e1e;
  --main-calculator-color: #1f1f1f;
  --calculator-hover-border-color: #181818;
  --calculator-box-shadow: 20px 20px 60px #1a1a1a, -20px -20px 60px #1a1a1a;
  --calculator-display-box-shadow: inset 8px 8px 18px #1c1c1c,
    inset -8px -8px 18px #222222;
  --calculator-button-box-shadow: 6px 6px 14px #1a1a1a, -6px -6px 14px #242424;
  --calculator-button-box-shadow-active: 4px 4px 14px #1a1a1a,
    -4px -4px 14px #242424;
  --toggle-theme-switch-icon-background: linear-gradient(
    145deg,
    #252525,
    #303030
  );
  --toggle-theme-switch-color: #323232;
  --toggle-theme-switch-box-shadow: 3px 3px 6px #1a1a1a, -3px -3px 6px #242424;
  --toggle-theme-switch-slider-box-shadow: 1px 1px 2px #1d1d1d,
    -1px -1px 2px #292929;
  --footer-color: #d0d0d0;
  --footer-link-color: #a0a0a0;
}

body {
  box-sizing: border-box;
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  font-family: 'Montserrat', sans-serif;
  background-color: var(--background-color);
  height: 100vh;
  margin: 0;
  transition: 0.4s;
}

.calculator {
  display: flex;
  align-items: center;
  justify-content: center;
  flex-direction: column;
  border: 1px solid var(--main-calculator-color);
  border-radius: 50px;
  background: var(--main-calculator-color);
  box-shadow: var(--calculator-box-shadow);
  height: 607px;
  width: 318px;
  padding: 32px;
}

.switch-container {
  display: flex;
  width: 100%;
  justify-content: flex-end;
}

.switch {
  position: relative;
  width: var(--toggle-theme-switch-width);
  height: var(--toggle-theme-switch-height);
}

.switch input {
  opacity: 0;
  width: 0;
  height: 0;
}

.slider {
  position: absolute;
  cursor: pointer;
  top: 0;
  left: 0;
  right: 0;
  bottom: 0;
  background-color: var(--main-calculator-color);
  box-shadow: var(--toggle-theme-switch-box-shadow);
  transition: 0.4s;
  border-radius: 0.9rem;
  border: 1px solid var(--main-calculator-color);
}

.slider:before {
  position: absolute;
  content: '';
  height: var(--toggle-theme-switch-icon-size, 1.4em);
  width: var(--toggle-theme-switch-icon-size, 1.4em);
  border-radius: 20px;
  left: 8px;
  top: 50%;
  transform: translateY(-50%);
  background: var(--toggle-theme-switch-icon-background);
  box-shadow: var(--toggle-theme-switch-slider-box-shadow);
  transition: 0.4s;
}

.switch input:checked + .slider {
  background-color: var(--background-color);
  box-shadow: var(--toggle-theme-switch-box-shadow);
}

.switch input:checked + .slider:before {
  left: calc(100% - (var(--toggle-theme-switch-icon-size, 1.4em) + 8px));
  background-color: var(--toggle-theme-switch-color);
}

.display {
  display: flex;
  align-items: flex-end;
  justify-content: space-between;
  flex-direction: column;
  font-family: 'Montserrat', sans-serif;
  font-weight: bold;
  letter-spacing: 3px;
  background: var(--main-calculator-color);
  box-shadow: var(--calculator-display-box-shadow);
  border: 1px solid var(--main-calculator-color);
  border-radius: 0.9em;
  width: 90%;
  height: 100%;
  padding: 16px;
  margin: 28px 0 28px 0;
  color: var(--color);
  overflow: hidden;
}

#operations-display {
  margin: 0;
  font-size: 21px;
}

#input-numbers-display {
  margin: 0;
  font-size: 42px;
}

.buttons {
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  gap: 15px;
  width: 100%;
}

.button {
  font-family: 'Montserrat', sans-serif;
  font-weight: bold;
  color: var(--color);
  font-size: 18px;
  cursor: pointer;
  border: 1px solid var(--main-calculator-color);
  border-radius: 0.9em;
  transition: all 0.2s;
  transform: scale(1);
  background: var(--main-calculator-color);
  box-shadow: var(--calculator-button-box-shadow);
}

.button:hover,
.slider:hover {
  border: 1px solid var(--calculator-hover-border-color);
}

.button:active {
  transform: scale(0.98);
  box-shadow: var(--calculator-button-box-shadow-active);
}

.zero {
  grid-row: 5;
  grid-column: 1/3;
}

#equals {
  grid-column: 4;
  grid-row: 4/6;
}

.square {
  aspect-ratio: 1;
}

footer {
  position: absolute;
  bottom: 0;
  font-size: 0.8rem;
  color: var(--footer-color);
  margin-bottom: 3px;
}

footer a {
  color: var(--footer-link-color);
  text-decoration: none;
}

footer p {
  display: flex;
  justify-content: center;
  align-items: center;
  gap: 5px;
}