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This is related to new dark looks for NetBeans implemented not so long ago (bug #225542). Currently there is a very small number of interface themes available, dark ones especially. The main reason of that state is difficulty in creating these themes. You need to write a plugin to do so, if I'm not mistaken. The idea is to provide the NetBeans community easier way to make mentioned themes than writing Java code, e.g. XML files optionally bundled with custom toolbar icons.