Not possible. Theme can only change the icons, not their arrangement.
The original theme imo is the most space-efficient. To change arrangement of icons you'd need to build LMMS from scratch and rearrange.
Recently there was an idea about having speedbars etc. auto-pop depending on mouse placement. I believe that sliding elements are a possibility in the graphic backend qT. That would indeed maximize workspace, but it would also get annoying fast, because slide-elements are not always responding.