Prepare alternate theme.json files

[kjellr]

As noted in the launch post, the theme has been designed to support a range of alternate color schemes. In the comps, these were also paired with fonts. I'm not sure that we'll have alternate color palette functionality built into WordPress by the time 5.9 is released, but that doesn't preclude us from building these out in the meantime.

I'll get started building these out as alternate theme.json files, to hopefully work with the explorations being done in WordPress/gutenberg#35619.

To test for now, I'm thinking the easiest way to build + test these would be to build them as child themes, and house them in the Theme Experiments repository. I'll start that way, and we can migrate them over to another form later on.

Screenshots

⏯️ Figma Prototype

[kjellr]

Adding to the Beta 1 milestone for now, but I definitely don't consider that a hard requirement.

[kjellr]

I opened a PR over here to try these out as child themes: WordPress/theme-experiments#292

[kjellr]

Work here is ongoing, and will be migrated over to Trac once the functionality is ready in Gutenberg. In the meantime, I'm going to close the issue here for now.