body:before {
background-color:
}
@dreki, just go to Appearance > Customize and look for Header and Sidebar Background Color and change from there.
@emil, thanks for pointing out this unusual, but useful feature.
@cannonpult, thank you! It’s not as obvious as it looks! I needed transparent background, but you can’t do that on most theme colour pickers (including twenty fifteen).
esmi will be here soon enough to tell you to start your own thread PositivelyRandy.