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

:root {
    --cell-size: 40px;
    --font-size: 32px;
}

body {
    margin: 0;
    padding: 0;
    display: flex;
    justify-content: center;
    align-items: center;
    min-height: 100vh;
    background: linear-gradient(to right, #00ffff, #adff2f);
    z-index: 900;
}

.container {
    display: flex;
    flex-direction: column;
    justify-content: center;
    align-items: center;
}

.board-container {
    display: grid;
    grid-template-rows: repeat(9, var(--cell-size));
    grid-template-columns: repeat(9, var(--cell-size));
    background-color: lightgray;
}

.cell {
    border: 1px solid black;
    display: flex;
    justify-content: center;
    align-items: center;
    font-size: var(--font-size);
    outline: none;
}

.cell-selected {
    background-color: rgba(255, 255, 255, 0.7);
}

.number-highlighted {
    background-color: rgba(255, 255, 255, 0.7);
}

.number-fixed {
    color: black;
}

.number-user {
    color: green;
    font-family: 'Franklin Gothic Medium', 'Arial Narrow', Arial, sans-serif;
}

.cell:nth-child(27n + 1),
.cell:nth-child(27n + 2),
.cell:nth-child(27n + 3),
.cell:nth-child(27n + 4),
.cell:nth-child(27n + 5),
.cell:nth-child(27n + 6),
.cell:nth-child(27n + 7),
.cell:nth-child(27n + 8),
.cell:nth-child(27n + 9) {
    border-top: 3px solid blue;
}

.cell:nth-child(73),
.cell:nth-child(74),
.cell:nth-child(75),
.cell:nth-child(76),
.cell:nth-child(77),
.cell:nth-child(78),
.cell:nth-child(79),
.cell:nth-child(80),
.cell:nth-child(81) {
    border-bottom: 3px solid blue;
}

.cell:nth-child(9n + 1),
.cell:nth-child(9n + 4),
.cell:nth-child(9n + 7) {
    border-left: 3px solid blue;
}

.cell:nth-child(9n) {
    border-right: 3px solid blue;
}

.numbers-container {
    display: grid;
    grid-template-columns: repeat(5, auto);
    margin-top: 20px;
}

.number-input {
    width: var(--cell-size);
    height: var(--cell-size);
    font-size: var(--font-size);
    color: green;
    font-family: 'Franklin Gothic Medium', 'Arial Narrow', Arial, sans-serif;
    background-color: lightgray;
    border: 1px solid black;
}

.number-delete {
    color: red;
}

@media only screen and (min-device-width: 360px) and (min-device-height: 460px) {
    :root {
        --cell-size: 40px;
        --font-size: 32px;
    }
}

@media only screen and (min-device-width: 640px) and (min-device-height: 700px) {
    :root {
        --cell-size: 60px;
        --font-size: 48px;
    }
}

.end-of-game-message {
    position: fixed;
    top: 0;
    bottom: 0;
    left: 0;
    right: 0;
    background-color: rgba(0, 0, 0, 0.9);
    display: flex;
    align-items: center;
    justify-content: center;
    flex-direction: column;
}

.winning-message {
    color: white;
    font-size: 3em;
}

.restart-btn {
    height: 50px;
    width: 100px;
    font-size: 1.2rem;
    border-radius: 2em;
    background-color: rgba(255, 255, 255, 0.7);
    outline: none;
}

.restart-btn:hover {
    background-color: rgba(255, 255, 255, 0.9);
    cursor: pointer;
}

.hide {
    display: none;
}