You are in the wrong location. Apply the changes here: Documents ▸ Publii ▸ sites ▸ YOUR_SITE ▸ input ▸ themes ▸ YOUR_THEME
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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