The style for the green button only applies to a woocommerce page, and the page with the shortcode is not a woocommerce page. So you’ll need to put the style in to apply to this page also. You won’t want to be editing the theme files in case they are overwritten by a theme update. Try this in your custom css:
.et-main-price {
background: #cff06b;
background: -moz-linear-gradient(top, #cff06b 0%, #c0dd65 100%);
background: -webkit-gradient(linear, left top, left bottom, color-stop(0%,#cff06b), color-stop(100%,#c0dd65));
background: -webkit-linear-gradient(top, #cff06b 0%,#c0dd65 100%);
background: -o-linear-gradient(top, #cff06b 0%,#c0dd65 100%);
background: -ms-linear-gradient(top, #cff06b 0%,#c0dd65 100%);
background: linear-gradient(to bottom, #cff06b 0%,#c0dd65 100%);
font-weight: 800;
padding: 8px 11px;
font-size: 14px;
-webkit-border-radius: 10px;
border-radius: 10px;
-moz-border-radius: 10px;
color: #fff;
text-shadow: 1px 1px 0 rgba(0,0,0,0.3);
-webkit-box-shadow: 0 1px 0 rgba(255,255,255,0.5) inset, 0 2px 1px rgba(0,0,0,0.1);
-moz-box-shadow: 0 1px 0 rgba(255,255,255,0.5) inset, 0 2px 1px rgba(0,0,0,0.1);
box-shadow: 0 1px 0 rgba(255,255,255,0.5) inset, 0 2px 1px rgba(0,0,0,0.1);
display: inline-block;
border: 1px solid #aed23f;
}
If your theme does not have a setting where you can enter custom css, you can use a plugin like this one:
https://ww.wp.xz.cn/plugins/simple-custom-css/
Have a look round the site to make sure no new problems are being created by this css.