Customization – How can error pages be customized?


When I google that, only information for customizing 404 pages is displayed. I apparently can not find a way to adjust this:

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I use a plugin that allows users to submit content in the frontend. While this error does not occur frequently, it does occur when the page is refreshed during submission. Can I customize the look in my child theme? I could not find any identifying elements, the body has only one class:

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