[TH] Holiday's includes custom style properties which can be edited in the style properties menu. You can navigate to this by going to Appearance > Style Properties > [TH] Holidays: CUSTOM_HOLIDAY.
These settings below allow you to add custom background colors, images, and freeform CSS/LESS code.
Disable holiday styling - This setting is based on user preferences which allows your forum users to disable the holiday styling. When enabled they are not able to see any holiday based styling/settings you have enabled. When disabled they can then see the holiday styling again.