I found a solution that I found on another topic.
The files are different since my version (Ext 6.5).
1) On directory ext\build\classic, copy directory theme "theme-neptune" (that's my custom theme) as "theme-neptune2" for example.
2) Inside this directory, rename file "theme-neptune.js" to "theme-neptune2.js"
3) Inside directory ext\build\classic\theme-neptune2\resources, rename file "theme-neptune-all.css" to "theme-neptune2-all.css"
4) On your project, go to MainModule and change property Theme to "neptune2"
That's all. No need to tell your users to delete cache on browser.