header {
color: green;
}

That’s exactly what I was looking for.

Why bother with individual modules? If you do it for every module, isn’t it the same as simply styling header in general?

I was just offering what I knew how to do. Thanks for pointing out what I didn’t know how to do. I did say, " I wouldn’t know how to do it universally" in the post you responded to. Yup, I did.

I don’t know if we agree on the term “header”. For me, the following changes the color of the text of every module-headline to green:

#custom.css
header {
    color: green;
}

(Note: .header does not!)

I’m looking for a simple solution if possible.

I don’t know if you consider this a simple solution, but you could add an entry for each module using the following.

body {
       margin:0px;
       height:100%;
       width: 100%;
       zoom: 110%;
       **color : white ;**
       position : absolute;
}

.MMM-Fortune .header {
  color: white;  /* any HEX color will do  */
}

.MMM-Fortune being the name of your module. One for each module’s header. I wouldn’t know how to do it universally, but you’re only talking about 10? modules. Copy and paste into your custom.css file a few times, change the name for each module, done.