Hey @marc , it’s my first OJS plugin, you’re talking with a noob here dude 
!
I did know about the addStyleSheet function defined in PKPTemplateManager. In fact, I used it during my first successful tests. My plugin was then a theme plugin, not a generic plugin.
But I found is that the JS & CSS we’re not displayed where I wanted, near the end of the <head>, but earlier. So my overwrite CSS files didn’t work quite well. I then went for the $additionalHeadData and I thought about having a folder with all modified or added files (CSS, JS and smarty templates).
I’m also upgrading our OJS to 2.4.6, so all became part of the same process. In a way, this plugin is the first step. I went with second one, an “override” template folder, and I asked in the forum here. Right know I think I’ve found a solution about what Alec said about compilation when using previously defined multiple template directories. I hope…
If I achieve to maintain a complete separate folder with all my modifications, upgrades will be much more easier to implement for me (before going into OJS 3, of course). So thanks a lot for the posted example, and even if I do agree that your way is easier, my goal is also to try to “package everything” but in a different way 
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Y así entre nosotros, Marc, he visto un poco lo has hecho y estás haciendo con el OJS y… ¡¡¡Qué crack 
!!!
Cheers,
Andy