This is related to the wiki: https://github.com/processing/processing/wiki/Dark-Theme-for-PDE

The wiki suggest to change the following files to create a Dark Theme:
theme.txt and preferences.txt

However, theme.txt file is located in the main Processing folder under the lib/ folder. If there is a new version of Processing, this file will be lost and to create the dark theme, it has to be changed again manually.

My suggestion: Move this file to the sketch folder to a folder called Themes, at the same level as the templates folder.

Kf