[Do not merge] Draft guidance doc for property pane custom control theme implementation

[sunyd0112]

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Learn Build status updates of commit d737d27:

✅ Validation status: passed

File	Status	Preview URL	Details
docs/spfx/web-parts/guidance/build-custom-property-pane-controls.md	✅Succeeded	View
docs/spfx/web-parts/guidance/neutral-theme-principles.md	✅Succeeded	View
docs/toc.yml	✅Succeeded	View

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[learn-build-service-prod]

PoliCheck Scan Report

The following report lists PoliCheck issues in PR files. Before you merge the PR, you must fix all severity-1 and severity-2 issues. The AI Review Details column lists suggestions for either removing or replacing the terms. If you find a false positive result, mention it in a PR comment and include this text: #policheck-false-positive. This feedback helps reduce false positives in future scans.

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Learn Build status updates of commit 2123fad:

💡 Validation status: suggestions

File	Status	Preview URL	Details
docs/spfx/web-parts/guidance/propertypane-theme-principles.md	💡Suggestion	View	Details
docs/spfx/web-parts/guidance/build-custom-property-pane-controls.md	✅Succeeded	View
docs/toc.yml	✅Succeeded	View

docs/spfx/web-parts/guidance/propertypane-theme-principles.md

• Line 0, Column 0: [Suggestion: filename-incomplete - See documentation] The filename contains an incomplete word, misspelled word, acronym, or abbreviation that is disallowed.

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Learn Build status updates of commit c8902a7:

⚠️ Validation status: warnings

File	Status	Preview URL	Details
docs/spfx/web-parts/guidance/build-custom-property-pane-controls.md	⚠️Warning	View	Details
docs/spfx/web-parts/guidance/property-pane-theme-principles.md	✅Succeeded	View
docs/toc.yml	✅Succeeded	View

docs/spfx/web-parts/guidance/build-custom-property-pane-controls.md

• Line 24, Column 120: [Warning: file-not-found - See documentation] Invalid file link: './propertypane-theme-principles.md'.
• Line 225, Column 146: [Warning: file-not-found - See documentation] Invalid file link: './propertypane-theme-principles.md'.

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[learn-build-service-prod]

PoliCheck Scan Report

The following report lists PoliCheck issues in PR files. Before you merge the PR, you must fix all severity-1 and severity-2 issues. The AI Review Details column lists suggestions for either removing or replacing the terms. If you find a false positive result, mention it in a PR comment and include this text: #policheck-false-positive. This feedback helps reduce false positives in future scans.

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[learn-build-service-prod]

Learn Build status updates of commit a5010c6:

✅ Validation status: passed

File	Status	Preview URL	Details
docs/spfx/web-parts/guidance/build-custom-property-pane-controls.md	✅Succeeded	View
docs/spfx/web-parts/guidance/property-pane-theme-principles.md	✅Succeeded	View
docs/toc.yml	✅Succeeded	View

For more details, please refer to the build report.

[andrewconnell]

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