Modern in the logo's colours, and the logo in the page's mode (#63)

Modern's palette was sampled from the 2004 iPodderX icon: a neutral navy,
its screen blue and amber EQ bars. It now takes the new logo's colours,
the dark half from logo-dark.svg (navy ground, #8fc2ea scale, #ff6a1a
needle) and the light half from logo.svg (sky ground, #2f6aa0 scale, the
needle taken down to #c43e00 so white on it clears AA). The pending amber
and the error red moved apart from the needle's orange, and the sign-in
page's copy of the palette follows.

The pages always showed logo.svg, the light variant, even in a dark
theme; logo-dark.svg was never served. It is now, and the app and admin
pages show whichever matches data-mode, dark until the script says light,
as the palette is. The sign-in page, which has no account's theme, picks
by the system's with <picture>.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -9,6 +9,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Changed
 
+- The Modern theme takes its colours from the new logo: its navy, the blue of its bars and the
+  orange of its needle.
+- The logo follows the page: the dark version in a dark theme, the light one in a light theme.
 - Between releases, the version on the logo says so: 0.9.1-dev, not 0.9.0.
 - The logo's needle is a brighter, redder orange that stands apart from the blue bars.
 - The logo is in the top bar beside the add-feed button, in place of the name at the top of the