applyTheme() mirrors the active mode onto <body> as data-theme="dark", but
the theme preset is only ever written to <html>. The palette token blocks in
tokens/colors.css and base.css used the bare attribute selector
[data-theme="dark"], so <body> matched them on its own and re-declared the
default dark palette (#1a1a1a / #2d2d2d / ...) directly on the body, where it
shadowed the preset values inherited from <html>. Every non-default preset
therefore painted the selected accent over the default preset's background,
surface, text and border tokens, and flipped into that state ~200ms after
load, when initTheme() first touched <body> — the accent-tinted background
flash seen on reload and nav-tab switches. Reached only in dark mode, since
light mode has no [data-theme="light"] token block.
Scope the palette token blocks to :root so a data-theme attribute on any
descendant (only <body> has one) can no longer re-declare them; descendant
rules such as [data-theme="dark"] .foo still match through <html>. Add a
regression guard that fails on bare attribute token blocks.
