hi Andy.

Which color do you want to change?

If you mean the background-color of Mo,Tu,We.. or Today,
you can change it by editing "Style for the flame of calendar".

Perhaps, the default value is "border:solid 1px green;".
If you want to change it into brown, use the style like "border:solid 1px black;background-color:brown;"

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The feature that can change its background-color is possible with newer than 0.60pre03.