In ACP > Manage > Privileges, select the category you want to allow guests to post in, and give guests the "Create Topics" and "Reply to Topics" privileges.
I'm looking to have a dark theme created for my forums. I will provide all the necessary information such as details, budget, deadline and more via PM. We're looking for someone who has at least 1-2 years of experience in HTML and CSS. If you are interested, please PM me.