It appears there is a conflict between your WordPress theme and the wpForo theme on your end. The WordPress theme is not dark, but the wpForo theme is, causing the TinyMCE editor to inherit colors from the WordPress theme.

To resolve this conflict, you can add the following custom CSS code in our settings. Navigate to "Dashboard > wpForo > Settings > Colors & Styles > Custom CSS":

.mce-container .mce-menu-item{
    color: black !important;
}

ok, saying "pb" i mean "problem".

The successive steps are :
1- we type a reply to a post
2- we want to change the police size character
3- we open and select an item in the barmenu police size, all characters are white on white background, so no readability.

We did not find where or how to change the color text in this option list (police size).

We would like to have black color as color text, and keep white color for background.

In the attach file, we can see i my screen and the liste open 

Greetings
Michel.

I'm really sorry but I cannot follow you. What do you mean saying "pb"?

What is the steps you're mentioning in the list? And which background should be white?

We use black color for background on the forum, lesamisguzzistes.fr

The pb is only when :
- we type a reply to a post
- we want to change the police size character or add an url link 
- we are getting menus with white characters on white background, so no readability.

We did not find where or how to change the color text in this option list (police size) or the boxtext (url link).

We would like to have black color as color text, and keep white color for background.

Greetings
Michel.