:root {
    --border-radius-full: 0.5rem;
    --border-radius-half: calc(var(--border-radius-full) / 2);
}

body {
    font-family: system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto,
        Oxygen, Ubuntu, Cantarell, "Open Sans", "Helvetica Neue", sans-serif;
    margin: 2rem;
}

h1 {
    font-size: 1rem;
    margin-block-start: 0;
    margin-block-end: 1rem;
    text-align: center;
}

button,
input {
    border: 0;
    color: inherit;
    font: inherit;
}

#settings {
    align-items: center;
    background-color: whitesmoke;
    border-radius: var(--border-radius-full);
    display: flex;
    gap: 1rem;
    padding: 0.5rem;
}

#settings * {
    border-radius: var(--border-radius-half);
}

#settings label {
    align-items: center;
    display: flex;
    gap: 0.25rem;
}

#settings input {
    border: 1px solid hsla(0, 0%, 0%, 0.2);
}

#settings button {
    background: rgb(77, 40, 26);
    color: white;
    margin-inline-start: auto;
    padding: 0.25rem 0.5rem;
}

#app {
    --color: black;
    border-radius: var(--border-radius-full);
    color: var(--color);
    display: flex;
    flex-direction: column;
    gap: var(--border-radius-full);
    padding: 0.5rem;
}

#carry-sums>*,
.addend>*,
#place-sums>*,
#addends::after {
    display: inline-block;
    font-size: 8vh;
    height: 2ch;
    padding: 0 0.25ex;
    text-align: center;
    width: min(7vw, 50px);
}

#carry-sums,
#addends,
#place-sums {
    text-align: right;
}

#place-sums,
.addend,
#carry-sums {
    display: flex;
    gap: var(--border-radius-full);
    justify-content: end;
}

#app input {
    background-color: transparent;
    border-radius: var(--border-radius-half);
}

#app input:user-invalid {
    background-color: pink;
}

#app input:user-valid {
    background-color: paleturquoise;
}

#app:not(:has(input:invalid)) {
    --color: white;
    background-image: url(fireworks.gif);
    background-position: center;
    background-size: cover;
}

#app:not(:has(input:invalid)) input {
    background-color: transparent;
}

#addends {
    border-bottom: 5px solid var(--color);
    padding-inline-start: 50px;
    position: relative;
}

#addends::after {
    bottom: 0;
    content: "+";
    display: inline-block;
    left: 0;
    position: absolute;
}