style.css

:root {
  --parent-width: 0;
  --parent-height: 0;
  --flat-width: 0;
  --flat-height: 0;
  --margin-top: 0;
  --margin-bottom: 0;
  --margin-left: 0;
  --margin-right: 0;
  --gutter: 0;
}

* {
  box-sizing: border-box;
}

body {
  margin: 0;
  display: grid;
  place-content: 'center';
  font-family: sans-serif;
  font-size: 1rem;
}

#root {
  display: flex;
  width: 100%;
  margin-inline: auto;
}

form {
  flex-basis: 320px;
  padding: 24px;
}

form > * + * {
  margin-block-start: 16px;
}

fieldset {
  background: #202020;
  color: white;
  border: none;
  border-radius: 8px;
  padding: 12px;
}

fieldset > * + * {
  margin-block-start: 16px;
}

legend {
  background: slateblue;
  font-weight: bold;
  padding: 4px 12px;
}

.control {
  width: 100%;
  display: flex;
  justify-content: space-between;
}

label {
  flex: 1;
  font-size: 14px;
}

input,
select {
  flex: 3;
  padding: 4px;
}

#canvas {
  flex: 4;
  background: #ccc;
  height: 100vh;
  display: flex;
  align-items: center;
  justify-content: center;
}

#parent-sheet {
  width: var(--parent-width);
  height: var(--parent-height);
  background: white;
  box-shadow: 0 1px 1px rgba(0, 0, 0, 0.12), 0 2px 2px rgba(0, 0, 0, 0.12),
    0 4px 4px rgba(0, 0, 0, 0.12), 0 8px 8px rgba(0, 0, 0, 0.12), 0 16px 16px rgba(0, 0, 0, 0.12);
  display: flex;
  flex-wrap: wrap;
  align-content: center;
  justify-content: center;
  padding-top: var(--margin-top);
  padding-bottom: var(--margin-bottom);
  padding-left: var(--margin-left);
  padding-right: var(--margin-right);
  gap: var(--gutter);
}

.flat-sheet {
  width: var(--flat-width);
  height: var(--flat-height);
  background: green;
  border: 1px solid black;
  padding: var(--bleeds, 0);
}

.flat-sheet::before {
  display: block;
  z-index: 99;
  content: '';
  background: white;
  width: 100%;
  height: 100%;
}