Hi it5,
Add below custom css code to add gradient background
#page {
background: -webkit-linear-gradient(#06B8D7, #fff);
background: -moz-linear-gradient(#06B8D7, #fff);
background: -ms-linear-gradient(#06B8D7, #fff);
background: -o-linear-gradient(#06B8D7, #fff);
background: -webkit-gradient(#06B8D7, #fff);
background: linear-gradient(#06B8D7, #fff);
}
Note: Change the above HEX color code #06B8D7
to your selected blue color.
Thank you!