Inline styles

<div style={{
    display: "flex",
    height: "53px",
    width: "fit-content",
    fontSize: "0.75em",
    alignItems: "center",
    flexShrink: 0            
}}>/* [...] */</div>

Custom CSS file

Simply create a Custom app-wide CSS. Make sure the class names are unique enough to not intersect with other UI elements, consider adding a prefix (e.g. x-mywidget-).