=> Note : You Can Change The background color by making an image and upload it to any Free Host ,
Then Find this code : body{background:url (your image url here)
change your url here with yours

exp:      

body{background:url(https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh0kTMl0cKzIUUPpIdCySh5-8FPHstgvoRyqpvAYCVhhmf-G-2fIML1XOVSWqQ8X54z-DEa4SvXtOq94oegs8-B8ViBpeSJy_wwSGnXSMUH0QxAavFus2DQj1FTTzzRCr-9uxX4FE9mj5Ii/s1600/background.png)

=>Note You Can do the same Trick To change the Post body background (if its an image)

By looking for this code  content-wrapper{background:url(your image url here)

=> If its not an image look for this code

body {
  color            : #000000;
  margin-left      : 20px;
  margin-right     : 20px;
  margin-bottom    : 20px;
  padding          : 0px;
  background-color : #FFFFFF;
  font-family      : arial, sans-serif;
  font-size        : 12px;
}

and change

color   : #ffffff to your desired color code