fix(theming): getColor shade default depends on transparency

[jzempel]

Description

On filling out the new website color principles page, it came to light that default shade fallback rules are as follows:

• without transparency: 700 light mode; 600 dark mode
• with transparency: 700 light mode; 500 dark mode

This PR makes those adjustments for the (highly minimal) edge case where color lookup is by hue only.

Checklist

• 👌 design updates will be Garden Designer approved (add the designer as a reviewer)
• 🌐 demo is up-to-date (npm start)
• ⬅️ renders as expected with reversed (RTL) direction
• ⚫ renders as expected in dark mode
• 🤘 renders as expected with Bedrock CSS (?bedrock)
• 💂‍♂️ includes new unit tests. Maintain existing coverage (always >= 96%)
• ♿ tested for WCAG 2.1 AA accessibility compliance
• 📝 tested in Chrome, Firefox, Safari, and Edge

[coveralls]

coverage: 95.657% (+0.001%) from 95.656%
when pulling c52a6c0 on jzempel/getcolor-defaults
into 23e010e on main.