ThemeKeys.PARCHMENT_CARD is a PanelContainer-base variation (only a
`panel` stylebox) — a Button looks up normal/hover/pressed/disabled,
which don't exist there, so a Button styled with it renders unskinned.
Add PARCHMENT_BUTTON, a Button-base variation with those four
styleboxes plus an empty focus box, built from Palette colours only.
Same fix pattern as DOCK_BUTTON for the system dock. Regenerated
game_theme.tres from the builder.
Phillip Tarrant
bca04e3a2e