You can either change the header's colors in CSS directly:
h1,h2,h3,h4,h5,h6 {color:red;}
or set the CSS to use the ColorPalette: (this is the default)
h1,h2,h3,h4,h5,h6 {color:[[ColorPalette::SecondaryDark]];
background:transparent;}
and then set the color by editing ColorPaletter tiddler:
SecondaryDark: #841