Foundry376/Mailspring · error · Error

Mailspring does not support stylesheets with the extension:

Error message

Mailspring does not support stylesheets with the extension: %@

What it means

cssContentsOfStylesheet was given a stylesheet whose extension is neither .less nor .css. Only those two formats are supported by the theme system; any other extension (or a path with no extension) is rejected instead of being read blindly.

Source

Thrown at app/src/theme-manager.ts:310

      const p = path.resolve(__dirname, stylesheetPath);
      return fs.existsSync(p) ? p : null;
    }
    for (const ext of ['css', 'less']) {
      const p = path.resolve(__dirname, `${stylesheetPath}.${ext}`);
      if (fs.existsSync(p)) return p;
    }
    return null;
  }

  cssContentsOfStylesheet(stylesheetPath) {
    const ext = path.extname(stylesheetPath);

    if (ext === '.less') {
      return this.cssContentsOfLessStylesheet(stylesheetPath);
    } else if (ext === '.css') {
      return fs.readFileSync(stylesheetPath, 'utf8');
    } else {
      throw new Error(
        localized(`Mailspring does not support stylesheets with the extension: %@`, ext)
      );
    }
  }

  cssContentsOfLessStylesheet(lessStylesheetPath) {
    try {
      const less = fs.readFileSync(lessStylesheetPath, 'utf8').toString();
      return this.lessCache.cssForFile(lessStylesheetPath, less);
    } catch (error) {
      let message = `Error loading Less stylesheet: ${lessStylesheetPath}`;
      let detail = error.message;

      if (error.line !== undefined) {
        message = `Error compiling Less stylesheet: ${lessStylesheetPath}`;
        detail = `Line number: ${error.line}\n${error.message}`;
      }
      console.error(message, { detail, dismissable: true });

View on GitHub (pinned to 648c685d60)

Solutions

1. Rename the stylesheet to .css or convert it to .less
2. Pass the path without relying on extension inference for non-style assets
3. Fix the theme/package manifest that references the unsupported file

When it happens

Trigger: Thrown at app/src/theme-manager.ts:310 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of Foundry376/Mailspring@648c685d60 (2026-09-03). Data as JSON: /api/errors/054b3b1842da42e5. Report an issue: GitHub.