fix(themes): an imported theme's bold and italic colours reach the editor (#676)

[PathGao]

What this is

Addresses #676, asked by @Fallgrim, whose note-taking depends on telling bold and italic apart at a glance: they had tried several VS Code themes and found that headings took colour while **bold**, *italic* and `code` never did, and concluded the editor could not style them.

After this, importing a theme that defines markup.bold / markup.italic / markup.inline.raw / markup.underline.link colours those spans in the editor. Measured with a probe theme (markup.bold.markdown: #ff0000, markup.italic.markdown: #008000, neither carrying a fontStyle), reading computed style off the rendered token spans:

span	before	after
**bold text**	mtk1 mtkb — default foreground, bold	mtk4 mtkb — rgb(255, 0, 0), bold
*italic text*	mtk1 mtki — default foreground, italic	mtk8 mtki — rgb(0, 128, 0), italic

The weight and slant survive because a theme entry without a fontStyle produces a rule without one, which Monaco reads as FontStyle.NotSet and merges with acceptOverwrite — the base theme's strong: bold and emphasis: italic stay standing. Spelling it regular instead would have flattened text the theme was only asked to colour.

Mechanism

parseAndApplyVscodeTheme copied each tokenColors entry's scope straight into a Monaco rule's token. Monaco resolves those against a trie of the names its grammars emit, and basic-languages/markdown/markdown.js emits strong, emphasis, variable for inline code and string.link — never markup.bold. So the rules were parsed, inserted under a token nothing produces, and never matched. Nothing failed and nothing logged; the theme just arrived partly applied.

Headings, lists and table pipes were the exception because they tokenize as keyword, which is also a TextMate scope themes define — the coincidence that made this look like "themes can only colour headings".

Scope

Only the four scopes Monaco can receive are renamed. ~~strikethrough~~, ==highlight== and ++insert++ are absent from the tokenizer entirely — it never leaves linecontent for them — so no rename reaches them; the preview renders all three (comrak's strikethrough, highlight, insert extensions), and closing that gap means owning a fork of the monarch grammar. Worth its own issue, not this change.

The built-in app-theme-light / app-theme-dark still ship rules: [], so out of the box the editor colours markdown exactly as before — this only unblocks imported themes. Giving the built-ins their own emphasis colours would change the default look for every user and is a design decision, not a defect.

Also left alone: the reporter's second ask, a place to add custom CSS. That is a feature request and belongs in its own issue.

The scope-splitting is in because it is the same defect one layer up: an entry written "scope": "comment, markup.bold" became a single token containing a comma and coloured nothing at all. Splitting it activates rules those themes always intended, which is a visible change for themes that use that spelling — and the direction the issue asks for.

Tests

scripts/markdownTokenColours.test.ts, five behaviour tests against monacoTokenRules, which this change extracts from parseAndApplyVscodeTheme so the rule construction can be run without a DOM or a Monaco instance. Drop the alias push and keep the tests: four of the five go red. The fifth is the "unusable entry is dropped" case, which is about the pre-existing guard rather than the fix.

Verification

npm audit             0 vulnerabilities
npm run check         801 files, 0 errors, 0 warnings
npm test              927 pass, 0 fail
npm run test:vitest   41 files, 365 pass
cargo test            148 pass

The before/after table was measured in Chromium against the dev server, with window.__TAURI_INTERNALS__ stubbed so the frontend boots outside Tauri, applying the probe theme through parseAndApplyVscodeTheme directly. Not measured: the import-a-theme-file path itself, which goes through Tauri and is unchanged here, and how any specific published theme spells its scopes — the prefix match covers markup.bold and markup.bold.markdown, but a theme that colours emphasis under some other scope entirely still will not arrive.