Editing textures using external editor

Some advice requested.

I often use the standard symbol for insulation in plan and section drawings that SU provides. The problem I find with it (and others) is that they have been drawn in a pale grey that does not print out very well and can be hard to see. I would like to change it to a darker grey or black.

I have set up Gimp as my image editor but when I click the icon in the dialog box, nothing happens. Is there another way of editing the texture? I am not sure where the file lives.

This what I see when I go to edit:

Screenshot 2023-07-31 at 10.22.14.pdf (132.8 KB)

Or maybe someone knows of some ready made architectural textures that can be downloaded that would solve the problem?