How can I add my own syntax definitions, themes and templates?

Go to the Application Support folder in the sandbox container

Textastic expects bundle and theme files to be in the Application Support folder in the app's sandbox container. These steps let you open the correct folder:

Open Finder. From the Go menu choose Go to Folder…

Enter the following path:
~/Library/Containers/com.textasticapp.textastic-mac/Data/Library/Application Support/Textastic

Click Go.

Copy custom syntax definitions and themes

Copy your .tmbundle TextMate bundles and .tmTheme themes into the directory.

Restart Textastic

Bundles and themes are loaded on application startup. Please quit and restart the app for changes to take effect.

After you restart the app, new syntax definitions can be chosen from the status bar of a document.

You can select added themes in the preferences screen.

Adding custom templates

You can also add your own templates to the "New from Template" menu. To create a new template, select File → New from Template → Textastic → Template.

This will create a new JSON file. The file is automatically saved in the template folder at ~/Library/Containers/com.textasticapp.textastic-mac/Data/Library/Application Support/Textastic/Templates

Changes you save to template files will be automatically reflected in the New From Template menu without restarting the app.

You can find a detailed description of the file format and example template files at https://github.com/blach/Textastic-Customization