You need to edit the CSS in store/webassist/themes/NM3/css/configurable.css for class 'productCart' (line 299) to change the color of the cart text.
Changing colors in the CSS is generally safe, but be aware that changing other properties could lead to unexpected results, as adjusting a class or ID property to get the desired results on one page may reflect badly on another page if that page has elements that use the same IDs or classes. Back up your pages and CSS files before making any major changes.
The reason that your change didn't 'take' is that the styles 'cascade' (Cascading Style Sheets) so your edits to lines that appear earlier on in the stylesheet will be negated by any rules that appear later on in that stylesheet.
Similarly, if more than one stylesheet is referenced in the header of your page, styles for IDs and classes with the same names in the later referenced stylesheet will supersede rules written in the earlier one(s).
If I were setting out to make significant changes to the CSS in pages that I did not design and create myself, I would be inclined to leave all the original stylesheets as they are, and create a new stylesheet (i.e. 'mystyles.css') and include that in the page headers after any existing css imports or includes, and add my own specific class and ID style rules to that stylesheet. That way, if things go pear-shaped, I know where to look for any errors and, if necessary, I can just revert to the original design rules by removing the references to my own stylesheet from the page headers.