In case it is of use to anyone, my Custom CSS for use with GeneratePress.
.is-form-style button.is-search-submit { margin-left:10px; height:3.5em;}
.is-form-style button.is-search-submit .is-search-icon{ height:3.5em;line-height:3.5em;padding:10px; width:46px;color: var(--contrast);background-color: var(--accent);border:0}
.is-form-style .is-search-submit path{fill: var(--contrast);}
.is-form-style button.is-search-submit .is-search-icon:hover{background-color: var(--highlight);}
.is-form-style.is-form-style-3 label {width: calc(100% - 46px)!important}
.is-form-style input.is-search-input {height: 3.5em; padding: 10px 15px;}