[useEuiTheme] Update colorMode return types

[thompsongl]

Summary

The accepted input and expected output values are (intentionally) different for colorMode values. inverse is an acceptable input value, but the output value accessible via useEuiTheme is the processed version of light or dark for consumer understanding.
The two types were incorrectly conflated and overly permissive. Now the consumer values will always be either light or dark, which will simplify creating component styles that take advantage of color mode switching.

Also note the change in EuiThemeColorMode that removes string as an option, instead adding the convenience lowercase versions of LIGHT, DARK, and INVERSE. We had left the door open for custom color modes, which flat out will not work with the light/dark concept foundational to EUI components. This is kind of a breaking change, but is more of bugfix in that attempting to use something like <EuiThemeProvider colorMode="custom" /> would break global styles.

All of this becomes more clear if reviewed alongside upcoming changes in #4575.

Checklist

• Check against all themes for compatibility in both light and dark modes
• Added or updated jest and cypress tests
• Checked for breaking changes and labeled appropriately
• A changelog entry exists and is marked appropriately

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_5639/

[chandlerprall]

Changes LGTM

This is kind of a breaking change, but is more of bugfix in that attempting to use something like <EuiThemeProvider colorMode="custom" /> would break global styles.

👍

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_5639/

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_5639/