Interface method -- get the value of a style attribute.
Theme.get(cls,pcls,attr): return value
- cls
class, for example "checkbox", "button", etc.
- pcls
pseudo class, for example "hover", "down", etc.
- attr
attribute, for example "image", "background", "font", "color", etc.
returns the value of the attribute.
This method is called from [[gui-style]].
print w.__class__.__name__, rect