When a replacement variable is inherited by a style, you may customize it in that style and have the changes automatically appear in any of its child styles.
To customize a replacement variable, simply click the [Customize] link next to any replacement variable on the main page.
This will load the Replacement Variable Customization form, which looks almost identical to the Add New Replacement Variable interface, and works in exactly the same way.
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After making your changes, the new value will be automatically inherited by any child styles.