ExcaliDraw: Make the background of a shape transparent (again)

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What I’m trying to do

Hi all, is there a way using the ExcaliDraw plugin to remove an assigned background colour to a shape, to make it transparent again? This is possible in the standalone version of ExcaliDraw. But I cannot find a method to do it in the plugin. I have to use the possibility to transfer the style from another (transparent) shape…
Did I overlook something?

Things I have tried