JSFiddle - React, Tailwind, and code Playground

HTML

<div id="container">
    <div id="header">HEADER</div>
    <div id="items">
        <div class="item" id="item1"></div>
        <div class="item" id="item2"></div>
        <div class="item" id="item3"></div>
        <div class="item" id="item4"></div>
        <div class="item" id="item5"></div>
    </div>
</div>

CSS

html, body {
    width: 100%;
    height: 100%;
    margin: 0;
    padding: 0;
    border: 0;
    overflow: hidden;
}


.item {
    background: green;
}



#container {
    display: flex;
    flex-direction: column;
    height: 100%;
}

 #header {
    flex-grow: 0;
    flex-shrink: 0;
    flex-basis: auto;
    height: 60px;
    background: red;
}

#items {
    flex-grow: 1;
    flex-shrink: 0;
    flex-basis: auto;
    display: flex;
    flex-direction: column;
}
.item {
    flex-grow: 1;
    flex-shrink: 1;
    flex-basis: auto;
    /* transition: all 2s; */
}

@media screen and (orientation: landscape) { 
   
    #items {
        flex-direction: column;
    }

 
}


@media screen and (orientation: portrait) {

    #items {
        flex-direction: row;
    }    
    
}


/* why does this not work?*/
.item:hover {
    background: orange;
}