Apply background color and modifiers from syntax themes

Catpuccin themes (e.g.
https://textmate-grammars-themes.netlify.app/?theme=catppuccin-mocha&grammar=rust)
seem to use italics, so added support for them. Don't see any themes
setting bgColor, but might as well.

Author: banga

src/highlightSyntaxInLine.ts

@@ -1,9 +1,35 @@
 import path from 'path';
 import * as shiki from 'shiki';
 import { FormattedString } from './formattedString';
-import { parseColorDefinition, ThemeColor } from './themes';
+import { ColorModifier, parseColorDefinition, ThemeColor } from './themes';
 export type HighlightedText = [string, ThemeColor | null];
 
+function parseShikiColor(token: shiki.ThemedToken): ThemeColor {
+    let modifiers: ColorModifier[] | undefined;
+    if (
+        token.fontStyle !== undefined &&
+        token.fontStyle !== shiki.FontStyle.NotSet &&
+        token.fontStyle !== shiki.FontStyle.None
+    ) {
+        modifiers = [];
+        if (token.fontStyle & shiki.FontStyle.Bold) {
+            modifiers.push('bold');
+        }
+        if (token.fontStyle & shiki.FontStyle.Italic) {
+            modifiers.push('italic');
+        }
+        if (token.fontStyle & shiki.FontStyle.Underline) {
+            modifiers.push('underline');
+        }
+    }
+    const themeColor = parseColorDefinition({
+        color: token.color,
+        backgroundColor: token.bgColor,
+        modifiers,
+    });
+    return themeColor;
+}
+
 export async function highlightSyntaxInLine(
     line: FormattedString,
     fileName: string,
@@ -23,14 +49,11 @@ export async function highlightSyntaxInLine(
         theme,
     });
 
-    let index = 0;
-    for (const { content, color } of tokens.flat()) {
-        if (color) {
-            const syntaxColor = parseColorDefinition({
-                color: color,
-            });
-            line.addSpan(index, index + content.length, syntaxColor);
-            index += content.length;
-        }
+    for (const token of tokens.flat()) {
+        line.addSpan(
+            token.offset,
+            token.offset + token.content.length,
+            parseShikiColor(token)
+        );
     }
 }

src/themes.ts

@@ -14,7 +14,7 @@ const THEMES_DIR = path.resolve(
  */
 type Color = string;
 
-type ColorModifier =
+export type ColorModifier =
     | 'reset'
     | 'bold'
     | 'dim'
@@ -25,7 +25,7 @@ type ColorModifier =
     | 'strikethrough'
     | 'visible';
 
-type ColorDefinition = {
+export type ColorDefinition = {
     color?: Color;
     backgroundColor?: Color;
     modifiers?: ColorModifier[];