Example

<div style=”background-color:blue;”>

You can use hexadecimal or rgb color instead if you prefer.

Cheers

article .content {
background-color:blue; !important
}

Go to Appearance>Customize then choose Additional CSS and add the above CSS.

Cheers

.background-image #wrapper {
  background: blue none repeat scroll 0 0;
}

That usually works. If not, try adding it in the Additional CSS part of your customizer. You could add the !important modifier if you still have trouble. Finally, element style attributes like jonpeck777 suggests will do it. These last two are usually avoided because they make it impossible for other CSS to override, which is also why they are sometimes useful.

This will not make the entire viewport background blue. For that set the body background. This is set as inline CSS, apparently by your parent theme. You’ll need to find where this is set to override it, it cannot be done in an external file. The only other way to override is adding the style attribute to the body tag on header.php.