Re-initialize the cached system colors so that widgets created or
repainted after this call pick up the correct dark/light theme
background and foreground colors.

GTK3 uses gtk_draw which calls gtk_render_background(context, cairo,
...). This asks GTK's live style context to paint the background — the
style context is always up-to-date with the current theme setting, so it
reflects dark/light dynamically without any cached values.

GTK4 removed the draw signal entirely. SWT replaced it with
snapshotBackground which uses Cairo-based custom painting via
drawBackground() → control.getBackgroundGdkRGBA() → defaultBackground()
→ the cached COLOR_WIDGET_BACKGROUND_RGBA. That cache is only populated
at Display creation time, so it goes stale when the theme is changed
afterwards.

Fixes Snippet393 on Gtk 4.20