Simplify Shrine theming

[guidezpl]

The dev docs have a bunch of slightly different Shrine theme implementations, which are also suboptimal due to changes in the theming system.

Write a single canonical Shrine theme and use it everywhere. (if Dartpad supported pub packages dart-lang/dart-pad#901, defining it once through one would ideal)

This should be somewhat easier after theming code is removed from .md files (#103)

[guidezpl]

material-components/material-components-flutter-codelabs#242 has the new Shrine theme implementation