Fix inverted gtk-dark.css in dark variants turning apps light when dark mode is requested

[AdrianColaianni]

Problem

The gtk-dark.css files shipped with the two dark variants (rose-pine-gtk and
rose-pine-moon-gtk) are color-inverted versions of the theme: the dark palette
was swapped into a light one (e.g. theme_bg_color is #f0f0f3 instead of
#191724/#1f1d2e).

gtk-dark.css is the stylesheet GTK3 loads whenever an application requests the
dark variant — via gtk-application-prefer-dark-theme=true in settings.ini,
GTK_THEME=rose-pine-gtk:dark, or apps that enable it at runtime when the
desktop reports color-scheme=prefer-dark.

The result is paradoxical: the more the system asks for dark mode, the more
these dark themes render light. Users end up with a light UI in GTK3 apps
despite having selected a dark theme and enabled prefer-dark.

Fix

For an already-dark theme, the dark variant is the theme itself. This PR makes
dist/gtk-dark.css an exact copy of dist/gtk.css for both dark variants, in
both the gtk-3.0 and gtk-3.20 trees, and rebuilds the corresponding
gtk.gresource bundles (the top-level gtk-dark.css files @import from the
compiled resource, so the bundles must match).

rose-pine-dawn-gtk is intentionally left unchanged: it is a light theme, so
having a dark gtk-dark.css is semantically correct.

Files changed

• gtk3/rose-pine-gtk/gtk-3.0/dist/gtk-dark.css + rebuilt gtk.gresource
• gtk3/rose-pine-gtk/gtk-3.20/dist/gtk-dark.css + rebuilt gtk.gresource
• gtk3/rose-pine-moon-gtk/gtk-3.0/dist/gtk-dark.css + rebuilt gtk.gresource
• gtk3/rose-pine-moon-gtk/gtk-3.20/dist/gtk-dark.css + rebuilt gtk.gresource

How to reproduce / verify

Before (theme installed, on any GTK3 app or a bare GtkWindow):

GTK_THEME=rose-pine-gtk:dark gtk3-widget-factory   # renders LIGHT

After this fix the same command renders the proper rose-pine dark palette
(#1f1d2e window background). Verified by reading dist/gtk-dark.css back out
of the rebuilt gresource bundles and by rendering a GtkWindow with the dark
variant forced.