I've only got some rough ideas for implementation.

I would suggest having a new class of templates with the format of something like `_layout.*.html`. By default the `menu()` method would try to use the `_layout.select.html` template for generating the appropriate html, and `field()` would use `_layout.input.html`. A user could override the template by using a `template` kwarg, ie. `menu(template='custom')` would use the `_layout.custom.html` template.

If no template can be found the default would be to fall back to the existing hard-coded html generation.

For long term maintainability it would probably be good to eventually replace the hard-coded html generated with a hard-coded template.

However these are just ideas, I haven't looked at how practical it would be to implement.