Create a block theme swapping tool

Without clear and followed standards for theme.json values, swapping between block themes is often a messy situation. Spacing, font sizes, and colors can be wildly different from theme to theme. Because so much of our content is now intertwined with the design, it's essential that we make theme transitions as smooth as possible.

One way of easing the transition would be to create a tool that lets users map values from an old and new theme and update them accordingly. For example, if your old theme had "slug": "large" you could map that to a closer alternative in the new theme, "slug": "30" and the values would be updated throughout the site/db accordingly.

Figma has a similar tool for fonts that is pretty intuitive. This interface could work fairly well here.

One limitation is there is no visual representation to see the difference between values so you can choose the best one. However, even as a developer tool, this would be quite valuable in helping us move the needle on block theme adoption.

No files, tests, or entry points are identified in the issue. Start by defining the mapping scope for theme.json values such as spacing, font sizes, and colors, then establish how mapped values should be updated throughout the site and database. Done means a usable theme-swapping tool exists with behavior aligned to the described Figma-style interface.