Restyling Through Code - fampat/Myra GitHub Wiki
Restyling through code is quite easy. There's a static object Stylesheet.Current that contains all default widget styles.
I.e. following code will make all Labels - green:
Stylesheet.Current.LabelStyle.TextColor = Color.Green;
Note. The default style is being applied to the widget in the moment of the creation. Therefore all changes to Stylesheet.Current should be done before the UI is created.