It appears that if one is using a theme (I’m using Ocean), then change one of the colors, say, in the Appearance tab of the Options dialog box, then decide to “Use Default” by clicking the box that says that, the default that’s returned to is the default for the default theme, not the default for the selected theme.
Bug? I would think so. It can be really hard to recreate a one-off color used by a theme if there’s no “revert to previous” or “undo this selection” choice available.
My guess is, this is true about the “Use Default” selection in general, but it’s especially destructive when it comes to color selections.