Sort theme variables in a more intentional order

[adamwathan]

This PR updates our default theme.css file and organizes the CSS variables in a custom order that's optimized for looking nice in Chrome dev tools.

You can only see ~50 variables "above the fold" in dev tools without clicking "Show all", so I've opted for putting variables near the top that have the highest "impact" on the design/brand of the site, and also showcase some of the modern CSS features we are using in Tailwind CSS v4.

Here's what it looks like:

I really want to show off the color palette because the swatches look nice and because seeing oklch there feels like a nice signal for Tailwind CSS v4 being very modern, but we have a lot of colors so you can't really fit much else if you do this. I stuck font families at the top since it's one of the few things that we can fit there that also feels important to your site's brand but still has few enough values not to push away the color palette.