Let's make NetBeans IDE look like this. Not saying it's a nice color or anything, just that it's possible to do so:

I changed the coloring in the Java editor by going to Tools | Options, then chose "Fonts & Colors", then selected the "Norway Today" profile and changed the background setting to Dark Gray.

Next, I put this themes.xml file into the "config" folder of the NetBeans IDE user directory, which you can identify as such by going to Help | About in the IDE. Go to the exact location defined by "User directory" in Help | About, and then go to the "config" folder within that folder:

The "config" folder of the user directory is the readable/writable root of the NetBeans IDE virtual filesystem. If a themes.xml file is found there, it is used, as described here.

Then, in netbeans.conf file, which is not in the NetBeans user directory but in the NetBeans installation directory, within its "etc" folder, I added the following to "netbeans_default_options":

-J-Dnetbeans.useTheme=true --laf Metal

The first of these enables usage of the themes.xml file, i.e., it notifies NetBeans IDE at startup to load the themes.xml file and to apply the content to the relevant UI components, while the second is needed because most/all of the themes only work if you're using the Metal Look and Feel.

Note: I must add that in most cases, whatever it is you're trying to achieve via a themes.xml file can probably be achieved in a different, and better, way. The themes.xml mechanism has been there forever, but is not actively supported or tested, though it may work for the specific thing you're trying to do anyway. For example, if you're trying to change the background color of a TopComponent, use the paintComponent method of the TopComponent instead of using a themes.xml file.