I am running PD Vanilla on XUbuntu 18.04. The system uses a dark theme. The problem is that PD tries to follow this dark theme but text and objects are barely readable.
Is there a way to tell PD not to observe the system theme settings ?

You can add lines of tcl code to the .tcl files. I had a similar problem with the file dialog windows, so I added this to pd-gui.tcl in proc init_for_platform:

option add *TkFDialog*foreground "black" userDefault
option add *TkChooseDir*foreground "black" userDefault

To change the canvas background color, you would add:

option add *PatchWindow*Canvas.background "red" userDefault

Unfortunately, if you want to change foreground color, that's specified in c, not tcl. If you're okay with performing a local build, you can check out my github fork of Pd, where I've implemented a dark theme.

@catherder-cowboy

Generally you can set the theme on a per application basis, Xubuntu/Xfce uses GTK which makes it fairly simple, this link seems to have a good amount of info and should lead you to an answer, been to long since I have played around with themes much so I can not offer much in the way of direct guidance. https://unix.stackexchange.com/questions/14129/gtk-enable-set-dark-theme-on-a-per-application-basis