The latest version of the Source theme does not have the loading attribute.
To add or remove this attribute in your theme, you need to edit the file, e.g. top.hbs or navbar.hbs available in the following location: (Documents ▸ Publii ▸ sites ▸ YOUR_SITE ▸ input ▸ themes ▸ YOUR_THEME ▸ partials).
PS: To preserve your custom code and prevent it being overwritten the next time the theme is updated, please ensure that you have created override files; these are, essentially, copies of the existing theme files that are stored separately from the main theme files → https://getpublii.com/dev/theme-overrides/
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