下記をコピーして追加CSSに入力


/* メルマガカスタム */
#custom-mail{
width: 90%;
margin-left: auto;
margin-right: auto;
}
#custom-mail br{
display: none;
}
#custom-mail input[type="text"], #custom-mail input[type="password"],
#custom-mail input[type="datetime"],
#custom-mail input[type="date"], #custom-mail input[type="month"], #custom-mail input[type="time"], #custom-mail input[type="week"], #custom-mail input[type="number"], #custom-mail input[type="email"], #custom-mail input[type="url"], #custom-mail input[type="search"], #custom-mail input[type="tel"], #custom-mail input[type="color"], select,#custom-mail textarea, .field {
display: block;
width: 100%;
margin-top: 1em;
margin-bottom: 1em;
padding: 0.5em;
border: 1px solid #bcbcbc;
border-radius: 3px;
background-color: #fff;
box-shadow: none;
color: rgba(0, 0, 0, 0.7);
font-size: 1em;
line-height: 1.5;
transition: ease;
}
#custom-mail ::placeholder{
font-weight: 500;
color: #b2bacc;
}
#custom-mail input:focus{
outline: none;
border-color: #f2253e;
}
#custom-mail textarea:focus{
outline: none;
border-color: #f2253e;
}
#custom-mail input[type="image"]{
transition: .3s all ease-out;
max-width: 100%;
}
#custom-mail input[type="image"]:hover{
transform: translatey(8px);
}

高度な設定→「CSS ID」に

custom-mail

と入力します