Keep OpenStation theme tokens out of the form previews

[AllTerrainDeveloper]

The bug

Previewing a form inside OpenStation let the desktop theme bleed into the preview: with a light form theme (e.g. Clean) under the dark official OpenStation theme, selects, inputs and textareas rendered near-black with white text inside an otherwise white preview card — in the builder canvas, the Theme tab and the Theme Studio.

Why

OpenStation's window-chrome.css themes raw controls in native window bodies:

.os-window__body :is( input[type="text"], …, select, textarea ) {
	background-color: var( --os-ui-surface, #fff );
	color: var( --os-ui-fg, #1e1e1e );
	border-color: var( --os-ui-border-strong, #949494 );
}

That weighs (0,2,1) — one specificity point over .atf-form .atf-input at (0,2,0) — so the shell's tokens won over the form theme's. The shell also claims ::placeholder at the same weight.

The fix

A shield block in builder.css, scoped to the preview surfaces (.atfs-preview__frame, .atfb-preview, .atf-block-preview), restating only the properties the shell claims with the form's own tokens at (0,3,0). It mirrors form.css's field-style ladder (outline / filled / underline / none, hover, focus) in the same source order, so the preview's internal cascade stays exactly the front end's. form.css — the public front-end stylesheet — is untouched.

Verified on the 8889 site (dark official OpenStation theme)

• Clean previews light: white select, dark text, the theme's own #8c8f94 border — canvas, Theme tab, Studio.
• Midnight previews with its own surface (rgb(29,35,39)), not the shell's (rgb(26,23,33)).
• The Filled field style keeps its --atf-surface-alt ground and transparent border, so the mirrored ladder preserves the variant cascade.
• Success-screen preview unaffected and correct.
• The standalone preview URL was already immune (separate document, pinned color-scheme: light).

🤖 Generated with Claude Code

https://claude.ai/code/session_01DVAaHFURDGeBzeuWHZ2dcP