:root {
  --color-primary: rgb(40, 40, 40);
  --btn-size: 75px;

  background-color: rgb(240, 240, 240);
  color: var(--color-primary);
}

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

body {
  margin: 0;
  font-family: sans-serif;
  background: linear-gradient(
    -45deg,
    #c0c0c0,
    #b0d0d3,
    #a3b8c2,
    #d8e6f3,
    #cfb898,
    #d2c59e,
    #c1f1d9,
    #f7b6b6,
    #cc99ff
  );
  background-size: 400% 400%;
  animation: gradientBG 50s ease infinite;
  transition: 1s ease-in-out;
}

@keyframes gradientBG {
  0% {
    background-position: 0% 50%;
  }
  50% {
    background-position: 100% 50%;
  }
  100% {
    background-position: 0% 50%;
  }
}

body.night-mode {
  background: linear-gradient(
    -45deg,
    #1a1a2e,
    #16213e,
    #0f3460,
    #1a1a2e,
    #2c3e50,
    #34495e,
    #22313f,
    #3b3b58
  );
  background-size: 400% 400%;
  animation: gradientNight 50s ease infinite;
  color: #f0e6dc;
}

body.night-mode button {
  color: var(--color-primary);
}

body.night-mode .calc-btn,
body.night-mode #ac-btn,
body.night-mode #backspace-btn,
body.night-mode #sign-btn,
body.night-mode #divide-btn,
body.night-mode #multiply-btn,
body.night-mode #minus-btn,
body.night-mode #plus-btn,
body.night-mode #equals-btn {
  filter: brightness(70%);
  transition: filter 0.3s ease;
}

body.night-mode .calc-btn:hover,
body.night-mode #ac-btn:hover,
body.night-mode #backspace-btn:hover,
body.night-mode #sign-btn:hover,
body.night-mode #divide-btn:hover,
body.night-mode #multiply-btn:hover,
body.night-mode #minus-btn:hover,
body.night-mode #plus-btn:hover,
body.night-mode #equals-btn:hover {
  filter: brightness(85%);
  cursor: pointer;
}

body.night-mode .calc-btn:active,
body.night-mode #ac-btn:active,
body.night-mode #backspace-btn:active,
body.night-mode #sign-btn:active,
body.night-mode #divide-btn:active,
body.night-mode #multiply-btn:active,
body.night-mode #minus-btn:active,
body.night-mode #plus-btn:active,
body.night-mode #equals-btn:active {
  filter: brightness(120%);
}

body.night-mode {
  background: linear-gradient(
    -45deg,
    #6a6a97,
    #5f77ba,
    #2d62a3,
    #8787d6,
    #6885a1,
    #4c6884,
    #5b748b,
    #9292be
  );
  background-size: 400% 400%;
  animation: gradientNight 50s ease infinite;
  color: #f0e6dc;
}

@keyframes gradientNight {
  0% {
    background-position: 0% 50%;
  }
  50% {
    background-position: 100% 50%;
  }
  100% {
    background-position: 0% 50%;
  }
}

section {
  display: flex;
  width: 100%;
  justify-content: center;
  align-items: center;
  min-height: 100vh;
}

#calculator {
  border: 2px solid var(--color-primary);
  background-color: #859ab1;
  padding: 8px;
  border-radius: 20px;
  box-shadow: 0 10px 15px rgba(0, 0, 0, 0.2);
}

#buttons {
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  gap: 8px;
  width: calc(var(--btn-size) * 4);
  height: calc(var(--btn-size) * 4);
}

button {
  all: unset;
}

button:hover {
  filter: brightness(110%);
  cursor: pointer;
}

button:active {
  transform: translateY(2px);
  box-shadow: inset 0 2px 4px rgba(0, 0, 0, 0.2);
}

button.focused {
  box-shadow: 0px 0px 0px 3px rgb(211, 211, 117);
}

.calc-btn {
  grid-column: span 1;
  grid-row: span 1;
  border-radius: 50px;
  border: 1px solid grey;
  text-align: center;
  background: #c3cfd9;
}

.calc-btn:active,
#ac-btn:active,
#backspace-btn:active,
#sign-btn:active,
#divide-btn:active,
#multiply-btn:active,
#minus-btn:active,
#plus-btn:active,
#equals-btn:active {
  filter: brightness(70%);
}

.calc-btn,
#ac-btn,
#backspace-btn,
#sign-btn,
#divide-btn,
#multiply-btn,
#minus-btn,
#plus-btn,
#equals-btn {
  transition: filter 0.5s;
}

#zero-btn {
  grid-column: span 2;
}

#ac-btn,
#backspace-btn,
#sign-btn,
#divide-btn,
#multiple-btn,
#minus-btn,
#plus-btn {
  background: #9fa0a1;
}

#equals-btn {
  background-color: #f5efc1;
}

#calc-display {
  height: var(--btn-size);
  border: 1px solid rgb(126, 126, 126);
  border-radius: 20px;
  margin-bottom: 16px;
  background: #b1bcb8;
  display: flex;
  flex-direction: column;
  justify-content: flex-end;
  align-items: flex-end;
  padding: 8px;
  font-size: 1.5rem;
}

.header {
  display: flex;
  padding-top: 1rem;
  padding-left: 2rem;
  padding-right: 2rem;
  justify-content: space-between;
  align-items: center;
}

.div-header {
  display: flex;
  align-items: center;
  gap: 1rem;
}

footer {
  align-items: center;
  font-size: small;
  text-align: center;
  padding-top: 1rem;
  padding-bottom: 1rem;
  font-size: 1rem;
}

.small-footer {
  display: flex;
  text-align: center;
  justify-content: center;
}

body.night-mode #calc-display {
  color: var(--color-primary);
}

#toggle-theme-btn {
  background: #425a90;
  background: linear-gradient(to right, #182848, #4b6cb7);
  color: white;
  padding: 10px 20px;
  border-radius: 30px;
  font-weight: bold;
  font-size: 1rem;
  border: none;
  transition: background 0.3s ease, transform 0.2s ease;
  box-shadow: 0 4px 8px rgba(0, 0, 0, 0.2);
}

#toggle-theme-btn:hover {
  background: linear-gradient(to right, #1d2b64, #5a6cbf);
  transform: scale(1.05);
  cursor: pointer;
}

#toggle-theme-btn:active {
  transform: scale(0.98);
  box-shadow: inset 0 2px 4px rgba(0, 0, 0, 0.3);
}

body.night-mode #toggle-theme-btn {
  background: linear-gradient(to right, #555, #222);
  color: #f0f0f0;
}

.find-out-link {
  background: linear-gradient(to right, #182848, #4b6cb7);
  color: white;
  padding: 10px 20px;
  border-radius: 30px;
  font-weight: bold;
  font-size: 1rem;
  text-decoration: none;
  display: inline-block;
  transition: background 0.3s ease, transform 0.2s ease;
  box-shadow: 0 4px 8px rgba(0, 0, 0, 0.2);
}

.find-out-link:hover {
  background: linear-gradient(to right, #1d2b64, #5a6cbf);
  transform: scale(1.05);
}

.find-out-link:active {
  transform: scale(0.98);
  box-shadow: inset 0 2px 4px rgba(0, 0, 0, 0.3);
}

body.night-mode .find-out-link {
  background: linear-gradient(to right, #555, #222);
  color: #f0f0f0;
}

#operators-explained {
  font-family: monospace;
  padding: 20px;
  text-transform: uppercase;
  letter-spacing: 1px;
  font-size: 2vh;
  padding-left: 5vw;
  padding-right: 5vw;
}

.addition,
.substraction,
.multiplication,
.division {
  padding-top: 5vw;
  padding-bottom: 1vw;
}

.addition {
  background-color: #4caf50;
  color: #fff;
  border: 4px solid #000;
  padding: 10px;
  margin-top: 12px;
  margin-bottom: 12px;
  box-shadow: 4px 4px 0 #000;
}

.substraction {
  background-color: #f44336;
  color: #fff;
  border: 4px solid #000;
  padding: 10px;
  margin-top: 12px;
  margin-bottom: 12px;
  box-shadow: 4px 4px 0 #000;
}
.multiplication {
  background-color: #ffeb3b;
  color: #000;
  border: 4px solid #000;
  padding: 10px;
  margin-top: 12px;
  margin-bottom: 12px;
  box-shadow: 4px 4px 0 #000;
}
.division {
  background-color: #2196f3;
  color: #fff;
  border: 4px solid #000;
  padding: 10px;
  margin-top: 12px;
  margin-bottom: 12px;
  box-shadow: 4px 4px 0 #000;
}

#calc-text.flash {
  animation: flash 0.2s;
}

@keyframes flash {
  from {color: rgb(40, 40, 40); }
  to {color: #b1bcb8;}
}