Post font color is WHITE by default

Is this in the Admin area or on the public site. If it’s the latter, then a link to your site would help.

It’s on the public site at GreenLifestyleIdeas.
The admin area is OK.

Something has inserted a huge block of CSS into your header. Possibly a plugin. Try deactivating all plugins and see if that helps.If this works, re-activate the plugins one by one until you find the problematic plugin(s).

I’ll try the plugins but the theme (atahualpa) was like this from the first, before I added any plugins?

Have you looked at the custom options for the theme?