Hello manunorsk,
Booking has default styles for default Joomla templates.
I don't know if you understand CSS but if your Joomla template has CSS with higher priority then Booking styles are replaced. We cannot affect it. It's question of improving styles of template.
Point two is French localisation. I checked out it and there is some problem with encoding.
If you open file: language/fr-FR/fr-FR.com_booking.ini you will see wrong characters as �
I don't know how repair it. I think it's possible only manually.