Texts supports custom templates in export to HTML format. Templates can contain any valid HTML 5 markup, including embedded CSS, and should contain $body$ as a placeholder for document body.
Files with templates for HTML should have extension .html5 and be stored in ~/Library/Application Support/Texts/ folder on Mac OS X or in %LOCALAPPDATA%\Texts folder on Windows.
Sample templates distributed with Texts: Default.html5, Bootstrap.html5.