Themes: Dracula, Material, Adwaita, Flat Remix, Paper, Nordic, each light, dark or Auto

The theme picker lists Modern (the old Dark and Light), Classic and six new
palettes, from Dracula's spec (with Alucard), Material 3's baseline scheme,
libadwaita's CSS variables, Flat Remix's _colors.scss, Paper and Nord. A second
setting picks Light, Dark or Auto where a theme has both; Classic and Paper do
not, so it is hidden for them.

The page gets data-mode, light or dark, and Auto is worked out in theme.ts from
the system, so each palette is written once instead of again under a media
query. Every new palette clears WCAG AA for text on its backgrounds. An old
ipx.theme of dark, light or auto carries over as Modern.

Closes #27.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

CHANGELOG.md

@@ -10,6 +10,13 @@ The long form, with what was wrong before and how it was found, is in
 
 ## [Unreleased]
 
+### Added
+
+- Six more themes in Settings: Dracula, Material, Adwaita, Flat Remix, Paper and Nordic, beside
+  Classic and Modern (the existing dark and light). Each that comes both ways has its own Light,
+  Dark or Auto setting; Classic and Paper come one way only, so that setting is hidden for them.
+  A theme chosen before this carries over.
+
 ### Changed
 
 - A feed that fails to check gets a red ! in the feed list, and its page says why, in place of a