Hi,
Thanks for testing Twistpad.
At the moment the syntax Highlighting is only enabled via the file extension of the opened document. However there are plans to make it easier to switch the syntax on the fly for any document.
In the mean time a tip that can help you is using the command File -> New and choose a document type.
If you have any ideas on what would be easier for you in switching the syntax type don't hesitate to post here.
Thanks,
André Santos Carthago Software Support
http://www.carthagosoft.net