Chart with virtual piano missing on android app ? (available on IOS)

[dflat]

Can someone tell if this is true or not, and if not how to enable it on Android ?

sorry it has not been implemented yet. Please use iPad as you have been doing

(also, I can't find how to change the background color of the paper sheet on Android)

There are limited choices for Android.
Settings>Color scheme - there are four choices.