To achieve Newsletter Block style like this.

#1. First, find Newsletter Block ID
You can follow this guide to find Newsletter Block ID

#2. Use this CSS code to Custom CSS.
#block-yui_3_17_2_1_1760947609783_24966 {
input.newsletter-form-field-element {
border: none !important;
padding-left: 0px !important;
background-color: transparent !important;
}
.newsletter-form-body {
border-bottom: 1px solid rgba(0,0,0,.12);
max-width: 450px;
display: flex;
justify-content: space-between;
align-items: flex-end !important;
}
.newsletter-form-fields-wrapper, .newsletter-form-fields-wrapper input, .newsletter-form-fields-wrapper>div {
width: 100% !important;
margin: 0 !important;
padding: 0px !important;
}
input {
padding-top: 5px !important;
padding-bottom: 5px !important;
font-size: 15px !important;
outline-color: transparent !important;
}
.newsletter-form-button-wrapper {
margin: 0 !important;
padding: 0px !important;
}
button * {
color: transparent !important;
}
button {
background-image: url(https://static1.squarespace.com/static/68f5739eac6e1d10ec79eecd/t/68f79c5e98e8705fb37444a3/1761057886186/Group+2.png);
background-size: contain !important;
background-repeat: no-repeat;
background-position: center;
padding: 10px 0px !important;
width: 50px !important;
}
.captcha-container.sqs-form-rendered.align-center {
display: none;
}}
