inconsisten behaviour of css (background, rgba)

Hello, i have a problem in customizing css of tundra.

STEP TO REPRODUCE
- I put a new background image on sooper_tundra/images/backgrounds.
- It correctly shows on backgroundimagekit configuration page.
- I choose it from the dropdownmenu, it shows.
- The color is different on the page because of the opacity on #container is set like this:

#container {
background:#fff;
background:rgba(255,255,255,0.5);
}

- so i open sites/all/themes/tundra/sooper_tundra/custom/style-custom.css and edit it like this:

#container {
background:rgba(255,255,255,0.0); /* tried also 255,255,255,0 without success */
width:80%;
}

the result is that the width is modified, not the opacity. (cleared cache many times, run cron many times)

INDEED:
- I get no effect even if i try to modify the style.css file in sites/all/themes/tundra/sooper_tundra/css/style.css
BUT
- in firebug i get effect modifing the rule in style.css, not adding a new rule on custom-style.

Help would be appreciate
Simone