Make behat steps overridable by themes.

Since themes can override the markup spat out on the page via renderers, the markup that is generated can end up breaking behat tests.

I would like to be able to run the entire behat test suite but with my custom theme being used throughout these tests.

At the moment the tests fail because my theme generates incompatible markup.
It is important to note that this markup is completely valid but different to what the behat tests require to run.

If themes could specify custom behat steps that overrode the default ones (in the same way that themes can override renderers) then it would be possible to get the full suite of behat tests to run successfully against a theme that significantly alters the page markup.