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This is in Textastic 3.0.
The new highlighter is now in Textastic 3.0 and supports TextMate syntax definitions and themes.
I've added a combined undo/redo button instead (segmented control) in Textastic 3.0.
You can add your own TextMate-compatible bundles and themes in Textastic 3.0.
New syntax highlighter with Monokai theme and the symbol list:
It's currently in beta and I'm soon going to send it to Apple for review.
In Textastic 3.0 you'll be able to use custom syntax definitions. There's a TextMate bundle available that you'll be able to use.