Dark themes have a habit of being bad at this, the way that in your screenshot there’s no distinction between panel title and panel contents, and intra- and int

Dark themes have a habit of being bad at this, the way that in your screenshot there’s no distinction between panel title and panel contents, and intra- and inter-panel borders are all the same. You’ve just generally got fewer grades to practically work with than in light themes, even if you don’t mess things up through a badly misguided sense of æsthetics (which they have). A light theme would be much more likely (though certainly not guaranteed, since the flat craze began) to have a light grey background on the panel title, a white background on the panel contents, and no border between the two (or at least a lighter grey than the inter-panel border), so that the only border (or at least the strongest border) would be above the panel (that is, above the panel title), making things much clearer. (The panel toolbar would also tend to get a light grey background, similar to the panel title’s or between that of the panel title and panel contents.)