<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def get_destination(self, filepath, targetdir=None):
 """ Return destination path from given source file path. Destination is allways a file with extension ``.css``. Args: filepath (str):
 A file path. The path is allways relative to sources directory. If not relative, ``targetdir`` won't be joined. absolute (bool):
 If given will be added at beginning of file path. Returns: str: Destination filepath. """

        dst = self.change_extension(filepath, 'css')
        if targetdir:
            dst = os.path.join(targetdir, dst)
        return dst