How to deal with margins around applets
Here's a fun choice: should applets themselves have the responsibility to draw margins around them, or should their container do that?
There are a couple of places where this issue arises. Firstly, we have the panel; if we draw the background of all applets "lightcoral", we can easily see that most applets draw some margin around them, though we still have the panel draw some margin between the applets themselves:
This can, IMO, create some issues. As an example, a nice idea would be for icons-only applets to have exactly a 1:1 aspect ratio (they currently don't); however, if we do that, the additional margin provided by the panel will be quite unnecessary (and, actually, might even feel excessive). However, if we allow applets to define the external margin themselves with little to no help from the panel, there's no way for themes to actually customize the density of the elements.
We have a similar problem with applets in dialogs. The theme will define an external margin to be used between the border of the dialog and the inner items. Usually, that amount is very small, and as a result, all applets will add some extra margin themselves. This causes some inconsistencies between applets (applying different amounts of margins) and makes it impossible for themes to have dialogs with no margins whatsoever (should they even be allowed to? I say yes, because why not).
My personal take, after a bit of time and thought, is that we should always explicitly ask applets (both compact and full representations) to always touch the external borders; if they set a certain size, they should always use all of it. This gives complete control of the external margin to the containment, which I feel is the easiest way to get both consistency and customization. This will require a bit of work on our own applets (I can do that) and asking for compliance from third-party applets (now is the best time to do that).
There are some scenarios where the applet reasonably can't actually touch the borders. As an example, all icons (for some reason which I'll never accept!) come with a margin built in directly in the SVG. In a utopia I would make all SVGs touch the external borders, but we can't feasibly implement this. Ideally we could read the actual size of the SVG element inside the file and base the size of the applet on that. Worst case scenario, we allow that SVG-icon margin to mess a bit with our design (as it does now, really). Buttons and other elements also come with built-in margin, but I'd say that's fine, since it actually gets used to e.g. display highlights and such.