I don't think you can make the envelope line thicker, but you can change the color of it. Go to Actions, type "tweak" into the filter and select Theme development: Show theme tweak/configuration window.
In the theme tweaker, type "volume" into the filter and double-click the color box for the Envelope: Volume to adjust the color. After you are done adjusting, save the theme (top right corner), preferably with your own name so the changes won't get lost later on.
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