Thank you for the explanation. I agree with you that it would be better to change the template style sheet instead of that of booking. So I tried first the change in the template style sheet. There was already such a style:
table {border-collapse: collapse;border-spacing: 0; width: 100%; border: 1px solid #ffffff;}

and I added background-color:#ffffff; at the end:
table {border-collapse: collapse;border-spacing: 0; width: 100%; border: 1px solid #ffffff; background-color:#ffffff;}

However I notice no change at all. Any idea how to solve the problem?

.

I have just realized that the calender tables are now white (see attachment and compare with previous one). There is surely another table (main table???) which should also be changed? Any idea which one?