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The built-in theme location is at {Arduino IDE installation folder}/lib/theme. Note it's "lib", not "libraries".
However, as long as you're using a recent version of the Arduino IDE, I recommend you take advantage of a new feature that allows you to install the theme in your sketchbook folder under the {sketchbook folder}/theme folder. You can find the location of your sketchbook folder in the Arduino IDE at File > Preferences > Sketchbook location. The reason it's better to install it there is because when you install it to the Arduino IDE installation folder you'll need to redo the theme installation every time you update to a new version of the Arduino IDE. When you install it to the sketchbook, the installed custom theme persists through IDE updates.