from typing import Union


def get_default_style(default_style_key: str, css_data: dict) -> Union[list, None]:
    """
    Retrieves the default style from the CSS list.

    Args:
        default_style_key (str): The name of the default style to use.
        css_data (dict): Dictionary of friendly style names and style source files.

    Returns:
        str|list: A string or list or strings pointing to CSS files.
    """
    default_style = None
    if default_style_key and default_style_key in css_data:
        default_style = css_data[default_style_key]
    return default_style