There is actually an invisible margin (or padding) on all sides of each skin. You can see the true size of the skin if you hold down Ctrl+Alt then left-click the skin. If you want less margin on the bottom, you'll also need to change the margin on the top (i.e., changing the bottom is not enough, unless you just want to reduce the space by half). Changing this margin might be a huge amount of work, partly because every skin will be affected, and also if there are any meters that are placed relative to the bottom of the skin. On the other hand, because raiguard used styles like he did, it's still a lot less work than if every skin had a separate background to change.
But in general, to alter the appearance, all you need to do is adjust the background of the skin. This is controlled by a style called , which you will find in the StyleSheet.inc file. (Changing this will affect every skin.) Some of the background logic also involves a meter that is positioned at the bottom of each skin, .
There is a key variable (also found in the StyleSheet.inc file). This controls the offset (or padding) used by the background style.
So to change the margin, first adjust the bgOffset variable to reduce it to the value you prefer. That will change all the margins equally. After that, you can decide whether you want to alter margins further by changing the style (which is actually a Shape meter), or tweak the meter to move it slightly, or both. Probably both.