BuddyPress.org

I used:

#bp-login-widget-submit {
border: 1px solid #78B8CE;
border-radius: 4px;
background: #84D0EF;
color: #fff;
}

and here is the result

It is much better than before, but the size of the button is always large. I can make the button smaller, but the link ‘Connexion’ will not be centered any more.
If I don’t have more chance with the bbpress widget, I will leave it with the code above.

Thank you very much for your help. 🙂