Textastic allows you to add your own custom syntax definitions and themes. You can add TextMate-compatible .tmbundle packages and .tmTheme files. You can also add custom file templates that appear in the file creation screen.
Go to the root folder in the Files section. Tap on the "+" button.
Choose "Folder" and enter "#Textastic" as the Folder Name. Tap "Done".
Make sure the newly created folder has a special icon as seen in the screenshot. The name of the folder is case sensitive.
Copy your .tmbundle TextMate bundles and .tmTheme themes into the newly created directory.
The new files are loaded on application startup. Please restart the app for changes to take effect.
To do this, leave the app by tapping on the home button once. Then open the task switcher by hitting the home button twice, and then hold down on Textastic until the icon starts jiggling. A red “-” should appear on the corner of the app icon. Press it to quit the app.
After you start the app, the new syntax definitions can be chosen in the File Properties popover. You can select added themes in the settings screen.
Starting with Textastic 4.0, you can also add your own templates that you can select in the file creation dialog.
You can find a detailed description and example files at https://github.com/blach/Textastic-Customization