In the custom CSS field provided by the plugin under Settings. I tried the following code, for example. The input field and submit button styles are accepted, but not the privacy text style at the beginning.
.tnp-widget tnp-privacy {
font-family: 'Open Sans', sans-serif;
font-size: 10px;
line-height: 1em;
min-height: 100%;
background: #FFF;
color: #333;
}
.tnp-widget input.tnp-email {
border: 1px solid #CCC;
background-color: #ffffff;
border-radius: 0;
font-family: 'Open Sans', sans-serif;
-webkit-appearance: none;
margin: 5px 0px 0px 0px;
padding: 0.3rem;
vertical-align: top;
line-height: 1rem;
}
.tnp-widget input.tnp-submit {
background-color: #6B7FA0;
box-shadow: 0;
border: 1px solid #7b9f35;
border-radius: 3px;
vertical-align: top;
border: none;
font-family: 'Open Sans', sans-serif;
font-size: 1em;
font-weight: 600;
cursor: pointer;
padding: 0.8em 1.5em;
color: #FFF !important;
margin: 0;
text-transform: uppercase;
-webkit-appearance: none;
text-decoration: none !important;
}
.tnp-widget input.tnp-submit:hover {
background-color: #536ea0;
border: 0px solid #536ea0;
}