Resolved.
Go to /templates/t3_bs3_blank/css and create a file, "custom.css"
Then copy this code:
.container.t3-header {
background-color: #chooseyourcolor;
width:100%;
margin-left: auto;
margin-right: auto;
}
This is for a background in full width.
If you don't want a full width background, copy this code:
.container.t3-header {
background-color: #chooseyourcolor;
}
To change footer background color, do the same thing with .t3-footer
or go to /t3-assets/css/css-58a73.css and change all background:#