space between buttons css

<style>
/* Added to highlight spacing */
.is-grouped {   
    display: inline-block;
    background-color: yellow;
}

.is-grouped > .button:not(:last-child) {
    margin-right: 10px;
}
</style>

Spacing shown in yellow<br><br>

<div class='is-grouped'>
    <button class='button'>Save</button>
    <button class='button'>Save As...</button>
    <button class='button'>Delete</button>
</div>