Create new "themeElements" capability [needs design doc]

[heath-freenome]

Prerequisites

• I have read the documentation

What theme are you using?

other

Is your feature request related to a problem? Please describe.

Many of the themes are nearly identical except for elements of the theme like grids, cards, control wrappers, etc...

Describe the solution you'd like

Create a new themeElements construct (similar to templates) that allows a theme to define things like GridRow, GridColumn, Card, Box, Divider, etc... as well as a way to provide properties to each of these concept at a field, template or widget level. @heath-freenome has a design in his head that really needs to be fleshed out.

Describe alternatives you've considered

No response