You can try taking the CSS of the reply button and transferring these properties to the add post button:
background: #00a3cc;
color: #fff;
Try adding this:
.header-buttons .btn.btn-default {
background: #00a3cc;
color: #fff;
}
.header-buttons .btn.btn-default svg {
color: #fff !important;
}
Next, you need to add css for :hover
(hover)
.header-buttons .btn.btn-default:hover {
background: #***;
}
Select the necessary colors more accurately. Replace *** with the color you want.
How to add css to Discourse:
You can do this with any part of css.