I have adjusted the CSS in custom.css file. This will not affect in any other page. It will only be applicable to offline page. This is needed to override template CSS because in template.css we have used different style.
.offline-inner #form-login input[type="text"],
.offline-inner #form-login input[type="password"]{
border-color: #157347;
color: #000;
padding: 20px;
margin: 10px 0px;
}
.offline-inner #form-login input[type="text"]::placeholder,
.offline-inner #form-login input[type="password"]::placeholder{
color: #000;
}
.offline-inner #form-login input[type="submit"]{
background-color: #157347;
}