afa2839a5a (Rename bg/fg palette keys to background/foreground)
aliased `color0` to plain background in theme color resolution, but
the Helix template still uses `color0` as the subtle surface shade
behind `ui.cursorline.primary`, `ui.virtual.ruler`, `ui.highlight`, and
`ui.text.focus`. All four now render at exactly the background color
and vanish: no cursorline, no ruler, and the picker's focused row is
only distinguishable by its bold text. This is the same failure that
d80c98f025 ("Make color0 distinct from background/foreground and fix
helix theme", #5538) fixed back when themes defined color0 directly.
Point those scopes at `lighter_background`, the semantic key for a
surface one step off the background. Every first-party theme defines it
distinct from background except Last Horizon and Solitude, which
set the two equal on purpose; those, and legacy themes whose
`lighter_background` falls back to background, render exactly as they
do today.
Assisted-by: Claude:claude-fable-5
Signed-off-by: Luke Hsiao <luke@hsiao.dev>
