[MaterialDatePicker] Dynamic Theme at runtime

I have a `MaterialDatePicker` and it correctly infers the colours from my theme. However, I'd like to customize the theming at runtime with dynamic values that I can't put into XML files.

Would it be possible to theme MaterialDatePicker when creating it?

Start at the MaterialDatePicker creation and theming APIs referenced by the issue. No files or tests are named, so first trace how the picker currently obtains theme colors and identify the supported path for runtime values. Done means the requested runtime customization is supported or its limitation is clearly documented, with coverage for the resulting behavior.

Written by the indexing model from the issue text.