* Use dark theme in GridTileBar

Reduce IconTheme, Typography hardcoding by applying a dark theme to the
contents. This is still not ideal; the theme brightness should really be
driven by backgroundColor.

* Add back IconTheme

Since IconTheme.of() finds the closest IconTheme, not the closest
IconTheme or Theme (which specifies IconThemeData), the previous commit
doesn't necessarily override any IconThemes futher up the tree.