webui: redable color of invalid fields on login-screen-like pages
    
    Pages with widgets like LoginScreen, MigrateScreen use login-pf styling.
    This page has dark background instead of light. Thus styling for labels
    for fields with error has color which makes the label hard to read or
    almost invisible.
    
    Change it to white so it is still readable.
    
    Fixes: https://pagure.io/freeipa/issue/7641
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>
    Reviewed-By: Petr Vobornik <pvoborni@redhat.com>
    Reviewed-By: Serhii Tsymbaliuk <stsymbal@redhat.com>