.content p, the .content part would be a theme dependent class for overall post content divs. Then style other blocks with selectors like .is-type-video. As needed to override the default p style, you can add custom CSS to individual blocks under the “Advanced” block meta in the right sidebar.
There’s really only three ways to add CSS if you haven’t built your own theme. One is the Additional CSS panel in the customizer. Another is to create a child theme with its own style.css file. Plugins can add CSS by using the wp_enqueue_style() function.
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