You see a dark band right above the track information panel ?
Is it possible to get rid of this ?
Also, this bug is consistent with all your skins.
This isn't a 'bug' with this or any of my skins. It's a feature of MusicBee that you will find showing up in any skin where the element 'HeaderBar' is set to a contrasting colour from the main panel. (The HeaderBar is the section along which the horizontal A-Z Bar, Section Titles, etc are shown.) When you select 'Hide Header Menu' for a panel, as you have done in your screenshot, it is only the text display that is hidden and the header background remains. As this element draws it's format from the HeaderBar element, that colour remains. In the case of ForumB, the HeaderBar colour is set to RGB 40,40,40 (Hex #282828) and this is the colour that is showing on your identified bar in the screenshot.
This is the same for older skins and an ideal one to see this is on 'Absolute Zero' which uses shades of lighter blue but it's equally visible on newer skins such as Alumni's Redstone Dark or diego's excellent 'Fluenx Dark'.
In answer to the question, "Is it possible to get rid of this ?". Yes. But only if you change the colour of the HeaderBar but this would mean that there would be no demarcation of the header section and the panel, just like having a screen of one single colour. Most skinners will have used the colour separation for an aesthetic reason. However, if you want to try it you could always write a simple xml file which states it depends on the original skin, and then set the HeaderBar colour to the same as the main panel. That should work on any skin, whether in straight xml format or compiled into xmlc format.