The neutral theme should be thought of as a very abstract theme, and should not typically need to be extended directly. Creating a custom theme by overriding "ext-theme-neutral" requires hundreds of variable overrides and many hours of work, and should only be done by very advanced theme developers, whereas a derivation of neptune or classic theme can be up and running in minutes by simply changing a couple variables. Additionally you can override "ext-theme-gray" or "ext-theme-access" if they provide a more desirable starting point for your custom theme.
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