To enter inspect mode and see what's going on, hit option-cmd-I (on a mac)
The theme source is (on my mac) located at:
I tried to widen the live preview editor window, but was not successful; it wasn't clear to me where in the CSS for my theme the CSS I was seeing in the inspector was coming from.
Changes to themes and snippets will be detected automatically and reapplied. You do not need to restart the app.