Details about a xml block text, for who needs to learn to read them:
A xml file has some similarities with html files. It has a start tag, and an end tag. For example, a xml file could start with a header, stating what type of file and which version, such as this one in rc.xml:
<?xml version="1.0" encoding="UTF-8"?>
Then it has a comment, which as such is not read by system (here openbox)
<!-- Do not edit this file, it will be overwritten on install.
Copy the file to $HOME/.config/openbox/ instead. -->
so in Geany text editor it is displayed in a grey color, which shows it is indeed commented and therefore not active.
A comment is a text between the 2 signs:
<!-- and
-->Then a first start tag:
<openbox_config xmlns="
http://openbox.org/3.4/rc">
and at the very end of the file, the end tag which closes the above tag:
</openbox_config>
the first tag contains an information which gives detail about "xmlns", but it could also have no information (I don't know what this information is used for... if needed I can ask other people knowing more xml than I do).
other tags are nested inside the main one above described.
Other tags nested, for example the tag for keyboard, as we are interested here about the multimedia keyboard:
The tag starts with:
<keyboard>
and ends with:
</keyboard>
and in between are nested a bunch of commands. Here the detail for one (the first I meet with just after the start keyboard tag):
<!-- Keybindings for desktop switching -->
<keybind key="C-Left">
<action name="DesktopLeft">
<dialog>no</dialog>
<wrap>no</wrap>
</action>
</keybind>
There is a comment, then at the line just after, a keybind key : if you press together Ctrl (symbolized by the C letter) together with the left arrow of the keyboard, you will start the action described at the following line : (action name = Desktopleft) display the next desktop available on the left (if you are not already using it).
the tag "dialog" ? No idea. The tag wrap ? same, no idea (if you guess... or if you know, I am always interested to learn more).
Each opened tag must be closed: <keybind key="C-Left"> is closed with </keybind> and the other informations/commands are nested in it. <action name="DesktopLeft"> is closed with </action> and the other infos are nested in it.
Each main action has it's own tag and the set of informations nested in it. This is why it does not seem too difficult to add the keybind keys directly in the file, once we know that, and after getting the explanation from this blog.
<keybind key="XF86HomePage">
<action name="Execute">
<command> google-chrome </ command>
</ action>
</ keybind>XF86HomePage will be replaced by whatever you find out with xev while pressing the button for the Mail, google-chrome by the Mail client you use... and you will create one full block text such as the one above for each button you want to configure on your keyboard.
You will create all these block texts just before the end tag
</keyboard> and you can even start with a comment to find it easily each time you need it, for exemple:
<!-- My own personal Multimedia --brand-model-- keyboard keys -->
What do you think ? Who wants to try and say how it goes ?
Well, I've just glanced at the article you linked to, so far. I'll give it a shot. But, probably not tonight. 
I wanted to add details about how to deal with the xml file, for anyone (unist maybe ?) who might need it. Who knows if I can also learn more...
Happy New Year ! It's 13 past midnight here !!!
