Hi !
I’m not sure what range of options you have on hosted forums, but I assume you have the Customize section in the admin panel.
In your theme, edit CSS/html and in Common add this:
.d-header .sign-up-button {
background: #48bb6a;
}
If it doesn’t work, you might need to add !important to make sure this color has the priority, like this:
.d-header .sign-up-button {
background: #48bb6a !important;
}