System administrators can manage any number of CSS, JS or font files that are included in all headers or footers. This includes the login page, form pages, and all other system pages. This allows you to further customise the design and behaviour of Publisher to suit your needs.
You will find these options stored within Themes:
Navigate to your profile dropdown > System admin:

Then navigate to Themes in System customisation:

The Themes screen is displayed:

Select which Theme you want to manage the JS & CSS file administration for, and click Javascript, CSS and Fonts:

In order to add a file to be used for JS, CSS and fonts, click Add file. The Add file screen is displayed:

Either Browse for the file(s) you want to use, or drag and drop. Once the files have been selected, additional fields will be displayed:

Select the location of the image, from the Location drop-down. Once you have selected your image, click Add.
The file will now be uploaded and viewable.
If you select a file with an invalid file extension, you will see the following error message:

Once you have added your files, they will be uploaded and active.