Primary links are not visible (of background color) to non-administrator users in a sub-theme

Good day everybody!

I am having the following problem when creating a subtheme of Newswire:
the Primary menu links in are visible to administrator users only. The links are there, but they are colored white, so not visible.

My subtheme is called Newswire-m. The folder structure is:

sites/all/themes/newswire
sites/all/themes/newswire-m
sites/all/themes/newswire-m/newswire-m.info
sites/all/themes/newswire-m/newswire-m-style.css
...

The file newswire-m-style.css is present and empty.
The file newswire-m.info reads ("..." stands for all the rest of regions, copied from parent theme's newswire.info file):

name = Newswire_M
description = Newswire subtheme
core = 7.x
engine = phptemplate
base theme = newswire
regions[leaderboard] = Leaderboard
regions[header] = Header box
...
stylesheets[all][] = newswire_m-style.css

Now when selecting Newswire-M as a current theme the primary menu links are visible for user with administrator rights only. The links themselves are there (mouse cursor changes when over them, and they are clickable). It seems that some styles are not inherited. In particular, in Firebug I see an element

#main-navigation {
background: url("images/red-nav-bar.png") no-repeat scroll left top transparent;
}

for an admin user, and this is not there for non-admins or anonymous.

Being not experienced in Drupal theming and CSS, I would highly appreciate any help or suggestion on this matter.

Thank you for sharing this fantastic theme!

Alex