I'm sorry but that's too bad. In this way wpForo saved the /classic-2/ folder in options. You'll have the same problem after each update. Somehow your wpForo installation doesn't have correct theme options. It seems you've made such changes before. Try to activate the /classic/ one. Click on the activation button of the one which comes from /classic/ folder, not from /classic_2/

Then delete all caches and do Ctrl+F5 on forum front. If it works, just delete the /classic_2/ folder.

I finally succeed to delete "classic_2" and to stay with only one theme named "classic" in the directory [ wpf-themes ].

Well ... I don't understand what was my problem.

So, finally, I will meet this problem each time I instal a new update of the forum, If I have a personnalised theme ?

With the news theme, it seems there is a problem of icon (cf copy screen)

If you currently use the /classic/ one you'll not have problem on update.

So, finally, I will meet this problem each time I instal a new update of the forum, If I have a personnalised theme ?

You're doing incorrect personalization. That's not correct to create a new theme in wpForo plugin folder. You should not create a new theme, this is not supported yet, you should use WordPress active theme folder . You should customize the existing theme using this instruction:

https://wpforo.com/docs/root/forum-themes/theme-customization/

However, we do lots of changes in 1.x.x versions in template files, so you should read Update Notes, if it says the template files must be updated you should update those after wpForo plugin update.

Okkkk.

I understand now why you asked me if there was a folder wpforo in my wp them !

What about my problem of icon ? Do you have an  idea ?

Put this CSS code in Dashboard > Forums > Settings > Styles > Custom CSS Code textarea. Save it, delete all caches and do Ctrl+F5 on forum front.

.fa-reply:before {
   content: "\f3e5" !important;
}