styles.css

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

* {
    margin: 0;
}

body {
    line-height: 1.5;
    -webkit-font-smoothing: antialiased;
    background-color: #e3e3e3;
}

img,
picture,
video,
canvas,
svg {
    display: block;
    max-width: 100%;
}

/* container style */
header {
    padding: 8px;
    text-align: center;
    border-bottom: 4px solid #aaa;
}

.container {
    display: flex;
}

.grid-container {
    flex: 4;
    display: flex;
    justify-content: center;
    align-items: center;
}

.grid {
    width: 600px;
    height: 600px;
    margin: 16px auto 16px;
    background-color: #fff;
    display: flex;
}

.menu {
    background-color: #dfdfdf;
    height: 100vh;
    display: flex;
    flex-direction: column;
    align-items: center;
    flex: 1;
    border-right: 4px solid #aaa;
}

.col {
    flex: auto;
    display: flex;
    flex-direction: column;
}

.cell {
    flex: auto;
}

.cell__border {
    border: 1px solid #aaa;
}

button {
    border: none;
    font-size: 1rem;
    cursor: pointer;
    width: 90%;
    margin: 4px 4px;
    padding: 12px 0;
    border-radius: 4px;
}

.active {
    background-color: #444;
    color: #fff;
}

.new {
    background-color: greenyellow;
}

.green-btn:hover {
    box-shadow: 2px 2px 5px green;
}

.btn:hover {
    box-shadow: 2px 2px 5px;
}

footer {
    padding: 8px;
    display: flex;
    justify-content: center;
    border-top: 4px solid #aaa;
}

.source-code {
    width: 32px;
    height: 32px;
}