PageColor property

Prior to v1.18.0 the rendered PDF page used the TPdfView's Color property as its background. That was awkward for dark-mode applications: setting Color := clBlack to blend the viewer into a dark UI also painted PDF pages on a black backdrop, which made any document with transparent regions look wrong.

PageColor decouples the two surfaces:

• Color — the scroll-area background, painted in the gap between the page rectangle and the control edge. Themed by the host application.
• PageColor — the PDF page background, painted inside the page rectangle, behind any rendered content. Defaults to clWhite.

The default clWhite means PDF pages render as paper-white regardless of the host application's theme — matching the look every other PDF reader gives. Applications that previously set TPdfView.Color to recolor the PDF page should set PageColor instead; the new default is a strict improvement for the common case.

• The page-render layer paints on top of PageColor — a page with opaque content shows that content; a page with transparent regions shows PageColor through them.
• Combine PageColor with PageShadowSize / PageShadowColor / PageBorderColor for the classic "white page on dark canvas with drop shadow" look.
• PageColor is a TColor — alpha is implicit ($FF). For semi-transparent overlays use a custom paint handler instead.