Theming is extremely easy to implement in java swing applications, it just requires adding the theme jar to the library path and a parameter to the command line to tell which theme it should use.
It seems that this method doesn't work with the exe launcher (the program freezes), but works fine if SH3D is started through the jar file.
A nice dark theme library can be found in flatlaf. Once you add the jar to the install_dir/lib folder, you can start SH3D from the jar file adding the following option to the command line:
-Dswing.defaultlaf=com.formdev.flatlaf.FlatDarkLafResult:
Here you can find the available themes in flatlaf:
https://www.formdev.com/flatlaf/themes/
On the same website you can download a nice theme editor to create your own theme (with real time preview).
