Update to theme to color-mode and update light & dark palettes.

[dbatiste]

GAUD-9068

This PR does a couple of things to prepare for updating the components:

• switch from theme to color-mode
• create the light & dark palette
• updates the demo-settings to "remember" last color mode preference

The palette is subject to change a bit as we proceed to update Daylight components. In addition, it is worth noting that we may eventually establish a new, more flexible primitive palette, and there are currently a couple of new hex colors used in the dark semantic palette that are not "set in stone" yet.

[github-actions]

Thanks for the PR! 🎉

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URL	https://live.d2l.dev/prs/BrightspaceUI/core/pr-6639/

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