Hi Gareth,
You're right, I removed their code from the css and it is still working.
About your question if they know what they are doing, I think you can better judge that than me. I do not have enough knowledge about this to make a reliable judgment. Maybe the information you gave me in your first reaction helped them to repair the error in the theme.
From a user of Moodle point of view, with not so much knowledge about the code, the response on my question was very fast from you but also from the designers of this theme. It also resulted in a solution.
Your comments and also the ones from Richard gives me some worries if I am on the right track with this theme. But we are to far in the configuration to change it to a other theme. This theme also offers some extra features which are very useful to use in courses.
Thanks again for your very fast support and the clear explanation.
Rene