This is a part of the intensity driven theme rewrite. 

It introduces the `toggle` and `interactive` helper functions to build
Toggle<T> and Interactive<T> styles for interactive elements in the
theme.

This PR also removes the `theme_testbench` crate and related actions.

Huge thanks to @osiewicz and @mikayla-maki for pushing this forward 🙏🏽

Release Notes:

- Updated the style of many interactive elements.