Macro define_theme 

macro_rules! define_theme {
    (NOTHING=) => { ... };
    (
        @ext_impls
        [ $head_ty:ident $($rest_ty:ident)* ]
        [ $head_field:ident $($rest_field:ident)* ]
        $name:ident ;
        $( $(#[$field_attrs:meta])* $field_name:ident : $field_ty:ty , )*
    ) => { ... };
    (
        @ext_impls
        [] []
        $name:ident ;
        $( $(#[$field_attrs:meta])* $field_name:ident : $field_ty:ty , )*
    ) => { ... };
    (
        $(#[$attrs:meta])*
        $(for = $for_ty:ident ; theme_field = $theme_field:ident ;)+
        $(%[component$($component_attr_control:tt)?])?
        pub $name:ident {
            $(
                %[fields$($cows_attr_control:tt)?]
                $(
                    $(#[$field_attrs:meta])*
                    $field_name:ident: $field_ty:ty,
                )*
            )?
    }) => { ... };
    (
        $(#[$attrs:meta])*
        $(%[component$($component_attr_control:tt)?])?
        pub $name:ident {
            $(
                %[fields$($cows_attr_control:tt)?]
                $(
                    $(#[$field_attrs:meta])*
                    $field_name:ident: $field_ty:ty,
                )*
            )?
    }) => { ... };
}