How to make Additional css changes global all across the web pages on WordPress website?

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I’m not sure exactly what the selectors in Astra or elementor but, you’ll have blocks of css like:

#post-423 > header > h1 {
    display: none;
}

If I wanted this specific rule to be sitewide I’d remove #post-423 > from that. If you provide us some of your custom CSS rules we could make a specific answer.