def style_category(category):
    """Return predefined Boostrap CSS classes for each banner category."""
    css_class = "alert alert-{}"
    if category == "warning":
        css_class = css_class.format("warning")
    elif category == "other":
        css_class = css_class.format("secondary")
    else:
        css_class = css_class.format("primary")
    return css_class