ds-site/Resources/Private/Sass/Backend/index.scss

.typo3-login {
    .panel,
    .panel-footer {
        background-color: rgba(0, 0, 0, .2);
    }
    input {
        color: #9CD9F0;
        border-color: #9CD9F0;
        background-color: #2E3436;

    }
    .form-control:focus {
        border-color: #77DFD8;
    }
    a {
        color: #fff;
    }
}