Custom themes are supported (although not in the evaluation version), however Sublime Text themes will need to be adapted to work in Sublime Merge: there's quite a bit more to the UI here, and the controls to target are quite different. Likewise with color schemes, as long as they define the new diff scopes, color schemes can be used, however none of the existing ones will yet. Syntax definitions, on the other hand, don't need any changes, and any you have installed for Sublime Text will be picked up automatically by Sublime Merge.

Plugin support is on the roadmap, and tabs will be added if that's what the user feedback asks for.

There's no dev channel yet, but we'll be adding one soon.