:root {
    font-family: "Segoe UI", "Helvetica Neue", Arial, sans-serif;
}

:root:not([data-theme]),
:root[data-theme="light"] {
    color-scheme: light;
    --surface: #ffffff;
    --background: #f2f3f5;
    --text: #1f1f1f;
    --border: #e1e4e8;
    --success: #46b450;
    --info: #2d7dfd;
    --danger: #d0342c;
    --result-surface: #f9fafb;
    --keypad-surface: #f5f7fa;
    --keypad-hover: #e9edf3;
    --shadow: 0 12px 28px rgba(31, 35, 40, 0.08);
}

@media (prefers-color-scheme: dark) {
    :root:not([data-theme]) {
        color-scheme: dark;
        --surface: #161b22;
        --background: #0d1117;
        --text: #f0f3f6;
        --border: #2d333b;
        --result-surface: #1f2733;
        --keypad-surface: #1c2733;
        --keypad-hover: #263042;
        --shadow: 0 18px 32px rgba(0, 0, 0, 0.48);
    }
}

:root[data-theme="dark"] {
    color-scheme: dark;
    --surface: #161b22;
    --background: #0d1117;
    --text: #f0f3f6;
    --border: #2d333b;
    --result-surface: #1f2733;
    --keypad-surface: #1c2733;
    --keypad-hover: #263042;
    --shadow: 0 18px 32px rgba(0, 0, 0, 0.48);
}

*,
*::before,
*::after {
    box-sizing: border-box;
}

body {
    margin: 0;
    min-height: 100vh;
    display: flex;
    justify-content: center;
    align-items: center;
    background: var(--background);
    color: var(--text);
}

.app {
    width: min(480px, 92vw);
    background: var(--surface);
    border-radius: 16px;
    padding: 32px 36px 40px;
    box-shadow: var(--shadow);
    display: flex;
    flex-direction: column;
    gap: 24px;
}

.app__title {
    margin: 0;
    font-size: 28px;
    font-weight: 600;
    text-align: center;
    letter-spacing: 1px;
}

.app__result {
    min-height: 64px;
    border: 1px solid var(--border);
    border-radius: 8px;
    padding: 16px 20px;
    font-size: 20px;
    line-height: 1.6;
    display: flex;
    align-items: center;
    background: var(--result-surface);
}

.app__controls {
    display: flex;
    gap: 12px;
    align-items: center;
}

.app__input {
    flex: 1;
    min-width: 0;
    height: auto;
    border-radius: 8px;
    border: 1px solid var(--border);
    padding-block: 14px;
    padding-inline-start: 12px;
    padding-inline-end: 12px;
    font-size: 16px;
    transition: border-color 0.2s, box-shadow 0.2s;
}

.app__input:focus {
    outline: none;
    border-color: var(--info);
    box-shadow: 0 0 0 3px rgba(45, 125, 253, 0.2);
}

.app__actions {
    display: flex;
    gap: 8px;
}

.btn {
    border: none;
    border-radius: 6px;
    padding: 8px 14px;
    font-size: 14px;
    font-weight: 500;
    color: #fff;
    cursor: pointer;
    transition: transform 0.1s, box-shadow 0.2s;
}

.btn:active {
    transform: scale(0.97);
}

.btn--success {
    background: var(--success);
}

.btn--info {
    background: var(--info);
}

.btn--danger {
    background: var(--danger);
}

.keypad {
    display: grid;
    gap: 12px;
    grid-template-columns: repeat(3, 1fr);
}

.keypad__key {
    height: 72px;
    border: none;
    border-radius: 12px;
    background: var(--keypad-surface);
    font-size: 26px;
    font-weight: 500;
    cursor: pointer;
    transition: background 0.2s, transform 0.1s;
}

.keypad__key:hover,
.keypad__key:focus-visible {
    background: var(--keypad-hover);
}

.keypad__key:active {
    transform: scale(0.96);
}

.keypad__key--zero {
    grid-column: span 2;
}

.theme-toggle {
    display: inline-flex;
    align-items: center;
    gap: 0.5rem;
    padding: 0.25rem;
    background: var(--surface-container-low, rgba(0, 0, 0, 0.04));
    border-radius: 999px;
    justify-content: center;
    margin: 0 auto;
}

.theme-toggle__label {
    font-weight: 600;
    letter-spacing: 0.5px;
}

.theme-toggle__option {
    display: inline-flex;
    align-items: center;
    gap: 6px;
    font-weight: 500;
}

.theme-toggle__button {
    display: inline-flex;
    align-items: center;
    justify-content: center;
    gap: 0.25rem;
    min-width: 4rem;
    padding: 0.55rem 1.1rem;
    border: none;
    border-radius: 999px;
    font-size: 1rem;
    background: transparent;
    color: inherit;
    cursor: pointer;
    transition: background-color 0.2s ease, color 0.2s ease, box-shadow 0.2s ease;
}

.theme-toggle__button:hover {
    background-color: rgba(0, 0, 0, 0.08);
}

.theme-toggle__button[aria-pressed="true"] {
    background-color: var(--accent-color, #0a84ff);
    color: #fff;
}

.theme-toggle__button:focus-visible {
    outline: 3px solid var(--focus-outline, rgba(10, 132, 255, 0.6));
    outline-offset: 2px;
}

.sr-only {
    position: absolute;
    width: 1px;
    height: 1px;
    padding: 0;
    margin: -1px;
    overflow: hidden;
    clip: rect(0, 0, 0, 0);
    border: 0;
}

@media (max-width: 540px) {
    .app {
        padding: 24px 20px 32px;
        gap: 20px;
    }

    .app__controls {
        flex-direction: column;
        align-items: stretch;
    }

    .app__input {
        height: auto;
        min-height: 56px;
        padding-block: 14px;
        font-size: 18px;
    }

    .app__actions {
        justify-content: flex-end;
    }

    .btn {
        flex: 1;
        text-align: center;
    }

    .keypad__key {
        height: 64px;
    }

    .theme-toggle {
        gap: 12px;
    }
}