If you have exactly the same issue, yes, you can use the same code. It solved my issue. But you'll have to change the background color according to yours.

Mine are green: #649E2D and #4caf50 are different green.

Can i use same code for my web site ?

More info here: 

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

2. or use the following CSS code:

#wpforo #wpforo-wrap .wpf-button:hover {
  background: #4caf50 !important;
}

Don't forget to change the red marked color code before using.

I see another CSS issue on your website. To solve it use this CSS code. Put it in Dashboard > Forums > Settings > Styles > "Custom CSS Code" textarea:

#wpforo #wpforo-wrap .wpf-button{
background: #649E2D !important
}

I have put the css code in the wrong place. It's now at the end of the file, and it works fine.

Many thanks for your help!

Have a nice day

I did the way you told me, with no result as you maybe can see, since you're logged on now.

That black is somewhat green, but i did not find it in the theme colors.

Please use the following CSS code, put it in your current active theme style.css ( Dashboard > Appearance > Editor > Stylesheet).

.colors-custom button{
   background-color: #fff !important;
}

.colors-custom:hover{
   background-color: #fff !important;
}

Delete all caches and check again. 

Here is a link: http://machinesetpaysans.fr/forums/public-forum/

And a screenshot "Add_a_Topic.png" is attached, showing the faulty buttons.

Thanks very much for your help.

Please leave your forum URL and describe more in detail (using some screenshots) what you need to change.