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Always, ALWAYS back up your custom CSS before upgrading your Emby server to a new version. Theme-breaking changes sometimes occur between releases, and having a backup ensures you can quickly restore your preferred appearance. Store your CSS in a text file outside your Emby server, preferably in a cloud-synced folder for additional safety. emby css themes portable
Inside the Emby Custom CSS box, reference your file using a local web server path if you reverse-proxy your assets, or use a direct relative system file path depending on your operating system architecture. However, the most robust offline portable trick is using an @import referencing a local Docker volume or absolute mapping that remains identical across your portable drives. Popular Open-Source Portable Emby Themes means: Always, ALWAYS back up your custom CSS