There aren’t any settings that can be used to adjust the calendar’s colors, but the colors can be overridden with a bit of CSS. I’m far from a CSS expert, but is something like this close to what you’re looking for?
The only change I’ve made is to darken the outer border and the lines that are used to divide the calendar days. If that’s the kind of change you are looking for, I can find the correct CSS selectors for you. (The adjustment I made in that screenshot was just done through my browser’s CSS tools.)