Hmmm… There has to be a global feature controlling the background pattern and color for your post. Lets try this, navigate to Appearance<<Customize<<CSS/JS Editor and use this bit of CSS to change your post background.
.entry-content {
background: #ffffff !important;
}
Hopefully this works for you but if not, you have access to our premium support through your web host (DreamHost). You can submit a Premium Support Ticket, which would give us access to your WordPress site so we can better assist you.