You can use https://github.com/SublimeText/InactivePanes/ for this. It does the job well enough to not require this be embedded into core imo. Once API Suggestions is implemented it will also work together with plugins like ColorHighlighter or basically any other plugin that modifies the active color scheme.
While I appreciate the plugin, it’s essentially a hack. The theme should be in control of what a focused or de-focused pane looks like. And indication of focus is so UX 101 core should be capable of this, imo.
wbond unlisted #3