Şunu bi dener misin hocam.
#bp-login-widget-form {
padding:.75em;
}
#bp-login-widget-form label,#bp-login-widget-form input {
display:block;
width:100%;
margin:0 auto;
}
#bp-login-widget-form input {
margin-bottom:1em;
background:initial;
border-bottom: 1px solid #cccccc69;
}