How to override a core translation in one theme ?

Hello,

After having created my custom theme, i want to customize some words or expressions that are already translated in the core .mo.
For example, in my case, "add items" is translated in French into "ajouter des éléments" in the module/code/po/fr_FR.po. But i want to translate it into "ajouter des images" in my custom theme (i don't want to modify the default translation).

I have tried to add this translation in my <my-theme>/po/themes_<my-theme>.po and compiled it, but nothing has changed. I'm sure that my custom translation works, because the translation works well when i add expression that not exists in the core .po .

So is it possible to override the core language translation with a custom theme translation ?

Thanks.