Accept and use style/theming configuration

## User Story
As a ODK project manager
I want to specify simple style customizations
So that my forms can match my branding

(Decide whether in scope/a separate issue)
As the developer of an application that uses ODK Web Forms
I want to specify a completely custom style
So that my forms can match the host application

## Questions
* Do we want to make it easy for people to opt out of Material/specify their own design language?
* What colors? Do we generate palette?
* Is passing in something like a logo image a theming concern?
* Do we allow for a preamble and suffix? Or is that a responsibility of the host app?
* Are we comfortable deferring the scope to whatever component framework we pick?
* If we defer to the component framework, should we still subset what we expose?
* Maybe styling is expressed as a CSS blob

The issue does not name any files, tests, or entry points. First resolve the open scope questions around Material, custom designs, colors, logos, preambles, and CSS blobs, then identify the component framework and theming entry points. Done should mean a defined, implementable theming configuration with agreed boundaries and acceptance criteria.

Written by the indexing model from the issue text.