Backwards Compatible Theme Changes

[cvarjao]

As theme keys gets refactor, new keys are added we need to provide a way to keep the same behaviour without breaking anything.

One solution might be to abstract theme definition and provide a accessor class which can then provide default values that preserves the existing behaviour.

[bryce-mcmath]

Needs some proposed changes

[jleach]

Brought this up at today Bifold WG call and directed people to the draft roadmap document. Next steps is I will produce a draft implementation to review and if approved we'll start pulling it through the application.