*_theme_contrast: implement

Contrast theme provides a large and contrast controls. Look [special.mos.ru](http://special.mos.ru) for example.
This may be useful for a users with reduced vision.

This should replace the nonsensical `simple` theme.

Start by locating the existing theme system and the implementation of the `simple` theme. Review the linked contrast-theme example and determine the required controls, then document what must replace `simple` and how the completed theme should support users with reduced vision.

Written by the indexing model from the issue text.