Not currently, though this is a question that gets asked a lot.
One of the benefits of Gestalt is that all styling is statically resolvable. This means that all it's CSS can be built to one small file or inline styles. We get a bunch of perf. & portability for free. (Unconstrained) theming requires runtime style resolution which would negate those benefits. There are also things that we'd like to be able to change and tweak globally and those get difficult if everybody is running separate themes. We recently switched to using system fonts and we're planning on bumping the color contrast.
However, we're generally _really_ open to constrained theming (i.e. more button colors).
