Welcome to the forums!
Custom Design is not just CSS code. :) But it's a big part of it right now. We put some work into changing site title, heading and body text fonts and size simple and that also includes access to 50+ fonts via Typekit.com included in the price of the custom design upgrade. We're working on adding more updates too! So your feedback and ideas are welcome.
I see that http://bethelisaharris.com/ is currently using the Elegant Grunge theme. In that theme, The CSS used to set the text color in the header looks like this:
#header div a, #header h1, #header h2, #blog-description {
color: lightGrey;
}
So, if you add that exact same rule to your Appearance → Custom Design → CSS page and change out the color code, that should work:
#header div a, #header h1, #header h2, #blog-description {
color: darkSeaGreen;
}
Here is a page with a list of colors: http://www.w3.org/TR/css3-color/#svg-color
You can also use HEX color codes like #CC0000 which happens to be red. See this page for an example list of basic HEX color codes: http://en.wikipedia.org/wiki/Web_colors#HTML_color_names