It's easy to add custom .tmTheme files to Textastic. Please have a look at this entry in the manual: http://www.textasticapp.com/v6/manual/lessons/How_can_I_add_my_own_syntax_definitions__themes_and_templates.html

https://github.com/Shopify/liquid-tmbundle

You can sign up for the Textastic 6 beta at http://goo.gl/forms/piRaSmBzmM. It has full support for the iPad Pro and split view multitasking.

Two days ago, an Apple certificate that is used to sign Mac App Store apps expired. This may require a login to the Mac App Store, so that a new receipt is generated that is signed with a new certificate. This should happen only once per machine.

Thanks, I can reproduce the issue. I'll fix it in an update.

Textastic 6 will support split view multitasking. You can sign up for the beta here: http://goo.gl/forms/piRaSmBzmM

It looks like the OneDrive file provider implementation is buggy. I had similar problems with it before and it was basically unusable. I hope they fix it soon.

Other apps like Working Copy or iCloud Drive work fine.