How can I set custom styles through the style getter?

[Krishuthapa]

static get styles(){
   return [ 
           css`  
           .flex {
            @apply --layout-horizontal;
            }`]
}

render(){
    return html`<div class="container flex">
                <div><span>Duration</span></div>
                <div><paper-radio-button>This Month</paper-radio-button></div>
                <div><paper-radio-button>Last 6 Months</paper-radio-button></div>
            </div>`
}

How can I set the style of the flex through the styles getter? If I do something llike the snippet above the styles aren't set