Images and theme for a Processing 4 beta. Navigate to Processing's installation folder, locate the lib folder, replace items as needed.
I do not specialize in making icons, logos or themes. If you don't like how the start, stop and debug buttons are colored or rasterized, I tried to recreate vector versions: see the 'vectorButtons` folder. (Don't copy these into the Processing installation folder, they'll not do anything.) Feel free to customize and improve.
You may wish to change the splash screen and icons as well. See Processing 4 alpha releases circa version 4. The more traditional serif font used by Processing is Enriqueta.
Syntax highlighting tag explanations can be found in processing/app/src/processing/app/syntax/Token.java.
To disable the "welcome screen," go to into your preferences and into defaults.txt and disable them.