General developer forum

I am not a CSS wizard and have never customised a theme for Moodle. But in general you would have to add the following line to wherever the attributes for the headers are defined:

color: white;

So if you want all <h1> elements in your HTML to have white text you would write:

h1 {color: white;}

I hope this helps a little bit. I would advise you to find a little course on CSS somewhere on the web.