How to edit the transparency of the grease pencil stroke?

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In my viewport, there is a stroke drawn lightly by the tablet pressure.
However, the transparency of the material assigned to the stroke is 1, so it is a fairly dark color.
Since the cause of this problem is the pressure applied to the stroke, not the set value of the material, I want to adjust the transparency by editing the properties of the stroke.
Q: How to access the transparency properties of already drawn strokes without touching the material??