Note about WordPress Separator block styles

wp-block-separator.md

To enable thicker separator rules while still using theme.json and being able to let authors set the color of the separator, do this:

      "core/separator": {
        "border": {
          "top": {
            "style": "solid",
            "color": "transparent",
            "width": "3px"
          }
        }
      }

Separator colors are applied as backgrounds. Thickness is set by border-top. Setting border-top to a solid, transparent rule extends the separator's size and let's the background color extend under it, creating a solid-color rule whose height equals the border thickness