Update Section Backgrounds article to show how to apply to FluentUI components

[PeterWCox]

Great article, but it would be nice to show how to apply the theme to FluentUI components.

It is not obvious to beginners that you can do this by using the 'theme' property on FluentUI components e.g. Buttons, Text etc.

Feel free to assign it to me and I will make the update

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Document Details

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• ID: d5fe832e-68cc-0f3a-acc1-c9116a5c981d
• Version Independent ID: 4b6961ad-45a6-aef9-f197-c31b1cb4a5f8
• Content: Supporting section backgrounds
• Content Source: docs/spfx/web-parts/guidance/supporting-section-backgrounds.md
• Product: sharepoint
• Technology: sharepoint-framework
• GitHub Login: @VesaJuvonen
• Microsoft Alias: vesaj

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