feat: allow for theme color fallbacks for overrides

[serikjensen]

This PR enables theme color fallbacks for overrides.

Without this change, the consumer would have to specify each color property individually. For example, even if they set colorErrorAccent, they would also have to set inputErrorColor, even though inputErrorColor uses the same value as colorErrorAccent.

This updates so that the partner could specify colorErrorAccent (or other colors) and the input colors would get those values by default. The consumer could then specify their own inputErrorColor if they had a distinct one, but this enables them to set less colors as a baseline if they don't want to fine tune.

This approach could be expanded to things like typography values, but focusing on colors for now.

We still use the return type of createTheme to type the theme properties. So we still automatically get updates to the theme type as properties are added either to createTheme or the base theme colors.

Proof of functionality

Theme configuration for colorBody:

Results in the following input backgrounds automatically

If we then specify a specific color value for inputBackgroundColor as well as a colorBody value:

The inputBackgroundColor takes precedence:

[jeffredodd]

LGTM