The customization S3 bucket associated with your application identifier should contain your customization bundled in a file named customization.zip.
The structure of the zip file content is
Default Files (folder)
style.css (optional)
icons (optional folder for icons that can be used in css, from custom modul and application.xml)
fonts (optional folder for fonts that can be used in css)
storageConfig.xml (optional, see https://graebert.atlassian.net/l/c/cwGm3Sy5 )
See https://graebert.atlassian.net/l/c/2JZdqwHo section for details about what can be customized and how to do it.