<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def hex_is_dark(hexx, percent=50):
 """ Function to decide if a hex colour is dark. Args: hexx (str):
 A hexadecimal colour, starting with '#'. Returns: bool: The colour's brightness is less than the given percent. """

    r, g, b = hex_to_rgb(hexx)
    luma = (0.2126 * r + 0.7152 * g + 0.0722 * b) / 2.55  # per ITU-R BT.709

    return (luma < percent)