Assuming that the editor never removes/adds/changes anything then yeah, it's basically free. But since most editors are somewhat of a moving target, it does take a bit of maintenance to make sure everything continues to look right as things update.

The worst part with maintaining a theme/colorscheme is that you can't really rely on automated testing to catch most of the issues, unless you start doing snapshot testing comparing PNGs or similar, and is the biggest time-sink when a new editor version been released.