[cre8ive-marketing]

Point 5 of #524, option (b), as approved.

What it does

Plain buttons (Revert, Browse…, Change…, Open folder …) had the same flat grey as a disabled field, such as the DPI box next to them. They now take the surface they sit on, with a thin border, so only a disabled control is a flat grey. Accent, ghost, icon and navigation buttons keep their own look.

• Soft.axaml: the Fluent ButtonBackground* / ButtonBorderBrush* resources in the light and dark dictionaries.
• Also under Material You and a theme.json surface: those themes replace SurfaceBrush, so ThemeService gives the buttons the same surface and hover colour. Without that, the buttons would stay white on a tinted card.
• Setup shares Soft.axaml, so its Change… button changes the same way.

Setup (Options)

The app (from the #524 discussion)

Tests

• New MaterialYouThemeTests.Plain_buttons_are_the_surface_with_a_border_unlike_a_disabled_box: under Soft and Material You, light and dark, a plain button resolves to the surface, hovers to the alternate surface and has a visible border.
• ThemeService_respects_custom_theme_json_overrides now also checks that a theme.json surface reaches the buttons.
• The SetupWindowTests screenshots above (29 pass).
• dotnet format --verify-no-changes is clean.
• Full dotnet test: Platform 81, Core 602, CLI 27, Setup 176, Engines 1821 and App 1345 pass, none fail.

Part of #524.

[cre8ive-marketing]

@KnifeLemon, CI is green on every check here. When you have reviewed it, could you merge it into main?

[KnifeLemon]

Thanks @cre8ive-marketing, this is (b) as approved in #524: plain buttons take the surface with a thin border in light and dark, Material You and theme.json surfaces follow through ThemeService, and Setup comes along since it shares Soft.axaml. The test pins the surface and border in both styles and variants. Merging.