Themes allow hunters to change the appearance of their Hunter's Journal (both on the Camp page and when seeing a shorter journal on other pages) and Hunter's Profile. By default, a hunter has access to the Classic MouseHunt theme, and other themes can be unlocked by having certain items in the inventory. Themes are selected by clicking on the hunter's profile (the shield to the left of the Camp button), then the Select Theme button. A list of themes appears, with a button for Select if the hunter has met the prerequisite, or a greyed out button Locked and a second line of explanation how to get the theme.
Great Winter Hunt Themes
Lunar New Year Themes