* {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
  font-family: "Gill Sans", "Gill Sans MT", Calibri, "Trebuchet MS", sans-serif;
  user-select: none;
}

/* ── LIGHT THEME (default) ── */
:root {
  --bg-page:        #f0ede8;
  --bg-cal:         #faf9f7;
  --bg-display:     #f5f2ee;
  --bg-field:       #ffffff;
  --bg-field-op:    #f5f2ee;
  --bg-reset:       #f5f2ee;
  --bg-equals:      #000000;

  --border-color:   #e0dbd3;

  --text-header:    #d2c2b7;
  --text-field:     #000000;
  --text-field-op:  #b68a76;
  --text-display:   #000000;
  --text-equals:    #ffffff;

  --shadow:         5px 5px 15px rgba(0, 0, 0, 0.3);

  --footer-color:    #000000;
  --footer-a:        blue;
}

/* ── DARK THEME ── */
body.dark {
  --bg-page:        #3a4764;
  --bg-cal:         #232c43;
  --bg-display:     #182034;
  --bg-field:       #2d3856;
  --bg-field-op:    #1e2740;
  --bg-reset:       #1e2740;
  --bg-equals:      #d03f2f;

  --border-color:   #1a2035;

  --text-header:    #ffffff;
  --text-field:     #ffffff;
  --text-field-op:  #637097;
  --text-display:   #ffffff;
  --text-equals:    #ffffff;

  --shadow:         5px 5px 15px rgba(0, 0, 0, 0.6);

  --footer-color:   #ffffff;
  --footer-a:       #e5e53c;
}

/* ── BASE STYLES ── */
#big-box {
  background-color: var(--bg-page);
  height: 100vh;
  display: flex;
  flex-direction: column;
  justify-content: center;
  align-items: center;
  transition: background-color 0.3s ease;
  gap: 50px;
}

#header-txt {
  display: flex;
  justify-content: space-between;
  align-items: center;
  font-size: 20px;
  color: var(--text-header);
  transition: color 0.3s ease;
}

#cal {
  background-color: var(--bg-cal);
  border-radius: 20px;
  padding: 20px 20px 30px 20px;
  display: flex;
  flex-direction: column;
  gap: 20px;
  width: 340px;
  box-shadow: var(--shadow);
  transition: background-color 0.3s ease, box-shadow 0.3s ease;
}

#theme-btn {
  border: none;
  background: none;
  cursor: pointer;
  font-size: 20px;
}

#small-box {
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  grid-template-rows: repeat(7, 40px);
  gap: 20px 25px;
}

#input-field {
  background-color: var(--bg-display);
  border: 1px solid var(--border-color);
  border-radius: 20px;
  width: 100%;
  color: var(--text-display);
  padding: 10px 10px;
  font-size: 45px;
  text-align: right;
  overflow: hidden;
  display: flex;
  align-items: flex-end;
  justify-content: right;
  outline: none;
  grid-area: 1/1/3/5;
  user-select: none;
  transition: background-color 0.3s ease, color 0.3s ease, border-color 0.3s ease;
}

.field {
  background-color: var(--bg-field);
  border: 1px solid var(--border-color);
  border-radius: 15px;
  display: flex;
  justify-content: center;
  align-items: center;
  font-size: 25px;
  user-select: none;
  color: var(--text-field);
  cursor: pointer;
  transition: background-color 0.3s ease, color 0.3s ease, border-color 0.3s ease, transform 0.1s ease;
}

.field:hover {
  filter: brightness(0.95);
}

.field:active {
  transform: scale(0.93);
}

/* operator keys */
#small-box > div:nth-child(2),
#small-box > div:nth-child(3),
#small-box > div:nth-child(4),
#small-box > div:nth-child(5),
#small-box > div:nth-child(9) {
  background-color: var(--bg-field-op);
  color: var(--text-field-op);
}

#small-box > div:nth-child(2) {
  font-size: 18px;
}

/* RESET key */
#small-box > div:nth-child(18) {
  background-color: var(--bg-reset);
  color: var(--text-field-op);
  grid-area: 7/1/8/span 2;
}

/* = key */
#small-box > div:nth-child(19) {
  background-color: var(--bg-equals);
  color: var(--text-equals);
  border: 1px solid var(--bg-equals);
  grid-area: 7/3/8/span 2;
}

@media all and (max-width: 391px) {
  #cal {
    width: 80%;
  }
}

footer{
  color: var(--footer-color);
}
footer>p>a{
  color: var(--footer-a);
}