LMMS is built using Qt and and it uses Qt's QMdiArea (MDI stands for Multiple Document Interface). Every window inside LMMS is a QMdiSubwindow, and they are not themeable using CSS. Their look is hard-coded in the Qt settings for your PC

A solution someone mentioned would be to actually render frameless windows and then draw a custom titlebar on top of them.

I origanally added the issue here HDDigitizerMusic#5, but @Umcaruje suggested that I add this issue on the main issues tracker.

Edited 2015-11-18 @tresf, added screenshot