style.css

* {
    box-sizing: border-box;
}

html {
    min-height: 100%;
}

body {
    font-family: "Montserrat", sans-serif;
    font-size: 1.6rem;
    margin: auto;
    max-width: 60rem;
    color: #d8edea;
    background: linear-gradient(#e66465, #9198e5) fixed;
}

h1 {
    text-align: center;
    margin-top: 20px;
    margin-bottom: 10px;
}

#game {
    display: flex;
}
#container {
    display: flex;
    justify-content: center;
}

#grid {
    width: 400px;
    height: 800px;
    display: flex;
    flex-wrap: wrap;
    margin-top: 15px;
    margin-left: 30px;
/*    outline: 1px solid white;*/
/*    border: none;*/
    cursor: none;
/*    border-radius: 10px;*/
}

#grid div {
    width: 40px;
    height: 40px;
}

.tetromino {
    background-color: blue;
}

#mini-grid {
    width: 160px;
    height: 160px;
    display: flex;
    flex-wrap: wrap;
    margin-top: 15px;
    margin-left: 30px;
    outline: 1px solid white;
    border: none;
    cursor: none;
/*    border-radius: 10px;*/
}

#mini-grid div {
    width: 40px;
    height: 40px;
}

#rules {
/*    border-radius: 20px;*/
    padding-block-end: 30px;
    display: flex;
    flex-direction: column;
    justify-content: space-around;
    align-items: center;
    text-align: center;
    margin-top: 1rem;
    width: 17.5rem;
    height: 25rem;
    background: linear-gradient(217deg, rgba(255,0,0,.8), rgba(255,0,0,0) 70.71%), 
                linear-gradient(127deg, rgba(0,255,0,.8), rgba(0,255,0,0) 70.71%), 
                linear-gradient(336deg, rgba(0,0,255,.8), rgba(0,0,255,0) 70.71%);
}

button {
    background-color: #f44336;
    border: 1px solid #222222;
/*    border-radius: 10px;*/
    box-sizing: border-box;
    color: white;
    cursor: pointer;
    display: inline-block;
    font-family: Circular,-apple-system,BlinkMacSystemFont,Roboto,"Helvetica Neue",sans-serif;
    font-size: 16px;
    font-weight: 600;
    line-height: 20px;
    padding: 13px 23px;
    position: relative;
    touch-action: manipulation;
    transition: box-shadow .2s,-ms-transform .1s,-webkit-transform .1s,transform .1s;
}

button:focus-visible {
    box-shadow: #222222 0 0 0 2px, rgba(255, 255, 255, 0.8) 0 0 0 4px;
    transition: box-shadow .2s;
    }

button:active {
    background-color: #F7F7F7;
    border-color: #000000;
    transform: scale(.96);
    }

button:disabled {
    border-color: #DDDDDD;
    color: #DDDDDD;
    cursor: not-allowed;
    opacity: 1;
    }

p {
    font-size: 16px;
    padding: 30px;
    text-align: justify;
    text-justify: inter-word;
}

a:link, a:visited {
    border: 1px solid #222222;
    background-color: #f44336;
    color: white;
    padding: 14px 25px;
    text-align: center;
    display: inline-block;
/*    border-radius: 20px;*/
    text-decoration: none;
    padding: 13px 23px;
    line-height: 20px;
}

#speed-control {
    display: flex;
    flex-direction: column;
    margin-left: 30px;
    width: 80px;
}

#speed-control * {
    padding: 8px;
    margin-top: 20px;
    text-align: center;
}

#speed-control span {
    border: solid 1px white;
/*    border-radius: 10px;*/
}