Update: Restrict edition of theme colors by users.

[jorgefilipecosta]

Supersedes #27082.
This PR follows a suggestion by @nosolosw in #27082 (comment).

With it:

• Users can add/update/remove their own new colors.
• Users can modify the color value, but not remove or change the name and slug of theme/core colors.

How has this been tested?

I made sure the behavior described above worked as expected.

[github-actions]

Size Change: +1 B

Total Size: 1.2 MB

Filename	Size	Change
build/components/index.js	172 kB	+1 B

ℹ️ View Unchanged

Filename	Size	Change
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build/annotations/index.js	3.8 kB	0 B
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build/autop/index.js	2.84 kB	0 B
build/blob/index.js	664 B	0 B
build/block-directory/index.js	8.72 kB	0 B
build/block-directory/style-rtl.css	943 B	0 B
build/block-directory/style.css	942 B	0 B
build/block-editor/index.js	133 kB	0 B
build/block-editor/style-rtl.css	11.3 kB	0 B
build/block-editor/style.css	11.3 kB	0 B
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build/block-library/editor.css	8.96 kB	0 B
build/block-library/index.js	148 kB	0 B
build/block-library/style-rtl.css	8.23 kB	0 B
build/block-library/style.css	8.23 kB	0 B
build/block-library/theme-rtl.css	792 B	0 B
build/block-library/theme.css	793 B	0 B
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build/components/style.css	15.3 kB	0 B
build/compose/index.js	9.95 kB	0 B
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build/data-controls/index.js	827 B	0 B
build/data/index.js	8.8 kB	0 B
build/date/index.js	11.2 kB	0 B
build/deprecated/index.js	768 B	0 B
build/dom-ready/index.js	571 B	0 B
build/dom/index.js	4.95 kB	0 B
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build/edit-navigation/style-rtl.css	881 B	0 B
build/edit-navigation/style.css	885 B	0 B
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build/edit-post/style-rtl.css	6.46 kB	0 B
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build/editor/style.css	3.85 kB	0 B
build/element/index.js	4.62 kB	0 B
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build/format-library/style-rtl.css	547 B	0 B
build/format-library/style.css	548 B	0 B
build/hooks/index.js	2.27 kB	0 B
build/html-entities/index.js	623 B	0 B
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build/list-reusable-blocks/style-rtl.css	476 B	0 B
build/list-reusable-blocks/style.css	476 B	0 B
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build/wordcount/index.js	1.22 kB	0 B

_{compressed-size-action}

[oandregal]

This works as advertised 🚢

[oandregal]

@jorgefilipecosta I've run into this behavior:

• add a user color with the name "my color"
• add a second user color with the name "my color"
• both are added to the color list, but the second overrides the first

This also happens in master so it has nothing to do with this PR.