style.css

/*
* CSS code for WordPress Login, Signup Page Design Tutorial.
* Author: WP Academy
& Tutorial Link: 
*/
.wpac-custom-login-form #um-submit-btn , .wpac-custom-signup-form #um-submit-btn {
	background: #242A56 !important;
	margin-top: 15px;
	border-radius: 0px !important;
}
.wpac-custom-signup-form #um-submit-btn {
	background: #8A4DEC !important;
}
.wpac-custom-login-form .um .um-form input[type=text], .wpac-custom-login-form .um .um-form input[type=tel], .wpac-custom-login-form .um .um-form input[type=password], .wpac-custom-login-form .um .um-form textarea {
	border: none !important;
	border-bottom: 3px solid #ccc !important;
	padding: 20px 20px 20px 40px !important;
	transition: 0.5s all;
}
.wpac-custom-login-form .um .um-form input[type=text]:focus, .wpac-custom-login-form .um .um-form input[type=tel]:focus, .wpac-custom-login-form .um .um-form input[type=password]:focus, .wpac-custom-login-form .um .um-form textarea:focus {
	border-bottom: 3px solid #242A56 !important;
}
.wpac-custom-signup-form .um .um-form input[type=text], .wpac-custom-signup-form .um .um-form input[type=tel], .wpac-custom-signup-form .um .um-form input[type=password], .wpac-custom-signup-form .um .um-form textarea {
	border: none !important;
	border-bottom: 3px solid #ccc !important;
	padding: 10px 20px 10px 20px !important;
	transition: 0.5s all;
}
.wpac-custom-signup-form .um .um-form input[type=text]:focus, .wpac-custom-signup-form .um .um-form input[type=tel]:focus, .wpac-custom-signup-form .um .um-form input[type=password]:focus, .wpac-custom-signup-form .um .um-form textarea:focus {
	border-bottom: 3px solid #242A56 !important;
}
.wpac-custom-signup-form .um-field-label {
	margin-bottom: 0 !important;
}

Merci pour ce super tuto