I will admit that I've gradually picked up a number of Christmas themed garments
- a plain navy jumper with a white polar bear on it
- a navy jersey dress covered in a gingerbread man print
- a cream man's t shirt with a bear wearing a scarf
- 2 blouses one cream, one navy with small Christmas prints (good for work)
There are a number of ideas online for taking a plain sweater/cardigan and applying removable decorations. I quite like the stars embroidered with contrast yarn.
https://www.youtube.com/watch?v=LaPLWd6bV7QI also have some Christmas earrings (holly/reindeer) and brooches (tree, snowflake, reindeer) which I add to a 'normal' outfit for interest.
I work in an office that does really like Christmas - there's a contest to decorate the bays.
For the craziest days I have a santa hat, elf hat or reindeer antlers.