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Themes can extend other themes using the "extends" configuration.  See User Interface Configuration for more examples examples.

Extending another theme means that you inherit all the settings of the extended theme.  So, if the current theme does NOT specify a component style, its ancestor theme(s) will be checked recursively for their styles before falling back to the default.  In other words, this "extends" setting allows for a theme to inherit all styles/components from the extended theme, and only override those styles/components you wish to override.

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• When the object at Handle '10673/2' (and any child objects) is viewed, the 'custom-B' theme will be used.  By default, you'll have the same styles as the extended 'custom' theme.  However, you can override individual styles in your 'custom-B' theme.
• When the object at Handle '10673/34' (and any child objects) is viewed, the 'custom-A' theme will be used. By default, your overall theme will be based on the 'custom' theme (in this case a "grandparent" theme).  But, you can override those styles in your 'custom-B' theme or 'custom-A' theme. 
  • The order of priority is 'custom-A', then 'custom-B', then 'custom'.  If a style/component is in 'custom-A' it will be used. If not, 'custom-B' will be checked .. and if it's there, that version will be used.  If not in either 'custom-A' or 'custom-B', then the style/component from 'custom' will be used.  If the style/component is not in ANY of those themes, then the default (base theme) style will be used.

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