The quickest way is to the use a docinfo file {1}. You can put any styles you want in that file. It gets included in the <head> tag of the HTML.
The shortcoming of CodeRay at the moment is that it doesn't have alternate themes out of the box (and we don't currently load a custom theme for it). Once we do, then you could just specify an attribute like `coderay-style` and provide your own theme.
p.s. FYI, why we made the font larger. The larger font size is kinder to your readers. Plus, the font size shrinks as the screen size gets smaller.