def minify_css(input_files, output_file):
    """
    Minifies the input CSS files to the output file.

    Output file may be same as input to minify in place.

    In debug mode this function just concatenates the files
    without minifying.
    """
    from .modules import minify, utils
    if not isinstance(input_files, (list, tuple)):
        raise RuntimeError('CSS minifier takes a list of input files.')
    return {'dependencies_fn': utils.no_dependencies, 'compiler_fn': minify
        .minify_css, 'input': input_files, 'output': output_file, 'kwargs': {}}
