import re


def normalize_rgb_colors_to_hex(css):
    """Convert `rgb(51,102,153)` to `#336699`."""
    
    regex = re.compile(r"rgb\s*\(\s*([0-9,\s]+)\s*\)")
    match = regex.search(css)
    while match:
        colors = map(lambda s: s.strip(), match.group(1).split(","))
        hexcolor = '#%.2x%.2x%.2x' % tuple(map(int, colors))
        css = css.replace(match.group(), hexcolor)
        match = regex.search(css)
    return css