programs.vscode.profiles.default.userSettings.workbench.colorTheme is
set unconditionally to the active palette's theme name (read from
themes/palettes/<theme>/apps/vscode.json), but the matching theme
extensions were bundled with devExtensions — which defaults to false.
So out of the box, VSCode silently fell back to the built-in dark
theme on every palette.
Split themeExtensions out as always-installed and devExtensions as
opt-in via nomarchy.vscode.devExtensions. themeExtensions covers the 6
palettes whose VSCode theme is packaged in nixpkgs (catppuccin,
catppuccin-latte, nord, tokyo-night, rose-pine, gruvbox).
The other 15 palettes (including the default summer-night, which uses
sainnhe.everforest) still break because their theme extensions are on
the VSCode marketplace but not yet in nixpkgs — handling that needs
pkgs.vscode-utils.extensionFromVscodeMarketplace plus per-palette
publisher/name/version/sha256 metadata. Logged separately.
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