Bonjour 
mes_ressources.txt
xmessage.maxHeight: 48
xmessage.minHeight: 48
xmessage.form.Thickness: 0
xmessage.form.message.background: yellow
xmessage.form.message.scrollVertical: true
xmessage.form.message.scrollHorizontal: false
xmessage.form.message.topMargin: 10
xmessage.form.message.leftMargin: 50
xmessage.form.message.rightMargin: 50
xmessage.form.message.cursorName: left_ptr
xmessage.form.message*Translations: #override <BtnDown>:
xmessage.form.message*font: -*-helvetica-*-*-*-*-25*
xmessage.form.message.textSink.foreground: magenta
xmessage.form.message.Scrollbar.borderPixmap: /usr/share/pixmaps/xterm-color_48x48.xpm
xmessage.form.message.Scrollbar.borderWidth: 48
xmessage.form.message.Scrollbar.scrollDCursor: left_ptr
xmessage.form.message.Scrollbar.scrollUCursor: left_ptr
xmessage.form.message.Scrollbar.scrollVCursor: left_ptr
xmessage.form.message.Scrollbar.width: 1
sed 's/^/-xrm "/;s/$/" \\/; 1s/^/xmessage -buttons "" -timeout 5 \\\n/; $s/$/\n"Magnifique message avec icon"/' mes_ressources.txt | bash -
Tu n'as pas parlé de xrdb pour ajouter tes ressources à la base.
Tu n'as pas parlé non plus de la variable d'environnement XENVIRONMENT qui peut charger un fichier spécifique.

Envoyé par
https://www.oreilly.com/library/view/x-window-system/9780937175149/Chapter09.html
Other Sources of Resource Definition
If xrdb has not been run, the RESOURCE_MANAGER property will not be set. Instead, the resource manager looks for a file called .Xdefaults in the user’s home directory. As we discussed earlier, resources found in this way are only available to clients running on the local machine.
Whether or not resources have been loaded with xrdb, when a client is run the following sources of resource definition are consulted in this order:
1. A file with the same name as the client application, in the directory /usr/lib/X11/appdefaults will be loaded into the resource manager.
2. Files in the directory named by the environment variable XAPPLRESDIR, or if the variable is not set, in the user’s home directory, with the name Class where Class is the class name of a client program.
3. Resources loaded into the RESOURCE_MANAGER property of the root window with xrdb; these resources are accessible regardless of the machine on which the client is running.
If no resources are loaded in this way, the resource manager looks for a .Xdefaults file in the user’s home directory; these resources are only available on the local machine.
4. Next, the contents of any file specified by the shell environment variable XENVIRONMENT will be loaded.
If this variable is not defined, the resource manager looks for a file named .Xdefaults-hostname in the user’s home directory, where hostname is the name of the host where the client is running. These methods are used to set machine-specific resources.
5. Any values specified on the command line with the -xrm option will be loaded for that instance of the program.
All of these various sources of defaults will be loaded and merged, according to the precedence rules described above in the section “Precedence Rules for Resource Specification.”
The client will then merge these various defaults specified by the user with its own internal defaults, if any.
Finally, if the user has specified any options on the command line (other than with the -xrm option), these values will override those specified by resource defaults, regardless of their source.
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