Standard theme is the Copenhagen theme that comes with Guide. It's a theme designed for best practices and mobile responsiveness, see Using the standard Copenhagen theme in Help Center. You can update the Copenhagen theme using branding and advanced settings in the Settings panel but you cannot edit the code to customize the theme. Otherwise, it will no longer be considered a standard theme.
When you use the standard Copenhagen theme for your help center, it is supported by Zendesk and automatically updated when new theme features are released.Note: Before November, 2016, Help Center contained five standard themes. If you are using one of these legacy themes, you can continue to do so, but you will not receive theme updates. If you switch your theme, you will not be able to switch back to the legacy theme.
When you create a custom theme, you take ownership of the theme. Custom themes are not supported by Zendesk and are not automatically updated when new theme features are released. You will continue to receive Guide features that are not theme-dependant, but you will not receive updates to your theme, as any update would overwrite your customizations. You can manually add new theming features. To stay informed about new theme features and to get instructions for adding them, subscribe to Zendesk announcements.
Marketplace theme (requires Guide Professional and Enterprise), is a theme created by Zendesk or a third-party that you can trial or buy from the Guide Themes area of the Zendesk Marketplace. Marketplace themes are supported by the developer of the theme and can be updated whenever a new version is released.
Themes come in three license types; trial, standard, and developer. You can't customize a theme with a trial or standard license. You can customize a theme with a developer license, but once you do, it becomes a custom theme and you can no longer download updates to it. However, if you make your changes to a copy of the theme, the original version can still receive updates.
For more information, see Using Guide themes from the Zendesk Marketplace.