You can set custom css via the Site administration > Appearance > Themes panel. Click the "Settings" icon for your theme (Boost in our case) and there should be a panel that let's you set you custom css. For Boost it's in the Advanced settings pane. You could try something like the following in your Raw SCSS
div.coderunner code {
color: inherit;
background-color: white;
font-size: 110%;
}
That gives you the following:

Be aware, though, that you're changing the appearance of <code> elements in CodeRunner questions for the entire site.