How to change draw signature dimensions and borders

Hello,
in Build Apps (free tier) I would like to change the style of the “Draw signature” box. Both Android tablet and smartphone render it quite small and the users are complaining that it’s hard to use, also because there is no border to show it better.
I am looking for a way to style the width, the height and the border (or at least the background colour).
Do you know how to do it?
Many thanks

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