Theme color improvements

[mdo]

This PR revamps our new text emphasis, subtle border color, and subtle background colors. Instead of manually using specific darker or lighter shades (e.g., $blue-300 for subtle primary border), they're now generated based on theme colors using the existing tint-color() and shade-color(). This is the more sensible approach given feedback in #37949.

This purposely avoids touching the grays for now—too annoying to tweak right now. Down the line in v6, I think we can revamp some theme colors, too:

• Change meaning of $secondary to allow for new body-secondary and body-tertiary approach in our CSS vars and utilities.
• Add an accent color that complements primary.
• Add emphasis and inverse buttons to replace light and dark.

Might be more coming for that as well.

Fixes #37949.