ONLY CHANGE THE ONES WITH TD STYLE. DO NOT CHANGE THE TR STYLE. If you do, it will break the code.
January is in this code here, but all you need is to edit the month and maybe the days.
Extra days from past month; color: rgb(255, 255, 255);
A normal day; #74FEB1
I suggest you not mess with this unless you want to change the color for a birthday.
You can find a live preview here. It needs to be updated, but you'll get the gist of it at the very least. Edit however you'd like, including whatever colors you'd like. All I ask is for proper credit because I spent a long time making this.