Create a directory, the name of which should contain only Latin letters and numbers, then create a file in this directory called `theme.yml`, and fill it with the following content:
Next, in `stylesheet.css` you can insert any CSS code, with which you can change the elements of the site. If you need additional pictures or resources, just create a `res` folder, and access the resources via the `/themepack/{directory name}/{theme version}/resource/{resource}` path.
To support the New Year's mood, which turns on automatically from December 1st to January 15th, create the file `xmas.css` in the `res` folder, and make the necessary changes.