media="all"
body {
background: #ececec url("../images/bg.png") left top;
}

I'm trying to change the value in the global.scss of my subtheme (created with drush) but I see no change.

I've changed the weight value 13 to 13 in the .info as I saw in a tutorial

; OPTIONAL STYLESHEETS

css[dlr_omega_style.css][name] = My sub-themes global style.
css[dlr_omega_style.css][description] = This file holds all the globally active custom CSS of my sub-theme.
css[dlr_omega_style.css][options][weight] = 12
settings[alpha_css][dlr_omega_style.css] = 'dlr_omega_style.css'

compass[status] = 1
compass[sass_dir] = "sass"
compass[css_dir] = "css"