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Themes aren’t migrated and need to be re-created manually using the Theme Editor.
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Have a look at the migration FAQ and the demos & other resources page.
And of course don’t hesitate to contact us:
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Migration assistance. Our support team is always there to help you with questions before, during or after migration.
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Step 7. Set up the site in Refined Sites for JSM Cloud
Read the post-migration guide which includes best practices and tips for user onboarding.