I don't know how it it solved for other templates but default and arctic does not have a style.css (probably mixed up with style.ini).
However, there you can adapt the color settings. Usually the color-variables are used widely and multiple times, not only on code blocks (e.g. __background_other__ and __background_alt__).
Another option is to adapt the design.css (which impacts the browser output but not print -> print.css) and enter the color directly into:
/* code blocks by code tag */
/* code blocks by file tag */
/* filenames for file and code blocks */
Example:
div.dokuwiki dl.code {
margin-top: 2em;
margin-bottom: 2.5em;
background-color: #686868; /* a darker grey */
color: #FFFFFF; /* white text color */
}
What ever will be the path you to follow, please remind yourself on the changes in the event of template updates.