#282 Custom styles for oriented widgets

[Joe English]

The inheritance mechanism for styles is intended to work so that if an application says something like: [ttk::button $b -style Custom.TButton] and the theme does not provide that particular style, it will fall back to the default TButton appearance.

This fallback mechanism doesn't work for oriented widgets:

ttk::scrollbar .sb -style Custom.TScrollbar
==> error "Layout Vertical.Custom.TScrollbar not found"
ttk::scrollbar .sb -orient horizontal -style
Custom.TScrollbar
==> error "Layout Horizontal.Custom.TScrollbar not found"