Add a new core theme

Description of the need

Since we are not making any progress on coming to a consensus about fixing CSS in Basis (https://github.com/backdrop/backdrop-issues/issues/4167), I'm inclined to begin to advocate for a new theme in core that we can eventually use as a default theme.

In my view, many UI or potential design improvements are on hold because of lack of any consensus on how to implement CSS changes to basis.

Proposed solution

Add a new theme to core that assume one of the following:

1) Take our time and try to get it right, recognizing that we will have the same inability to fix problems with this new theme, once it has been committed. 2) Make it clear that this is a DEMO theme designed to showcase Backdrop and that we will be making breaking changes to it. Users should only use this theme if they are willing to accept the risk or willing to fork it and use it like a custom theme (no updates). 3) Build this theme with the idea of supplemental stylesheets (https://github.com/backdrop/backdrop-issues/issues/4512) or supplemental selectors (https://github.com/backdrop/backdrop-issues/issues/4782) in mind from the outset.