I've modified (locally) theme-zen to have different css for .LRN admin pages, and also Site Wide Admin ones. First, I've added two parameters "AdminCssUrl" and "dotLRNAdminCssUrl" to theme-zen. By default, these parameters could have the same color than the current behaviour, in order not to bother to users who want to upgrade.
Then, adding a few lines in lrn-master.tcl to get those parameters and 2 "if" statements to choose if the url is about .LRN admin or <acronym title="site wide admin">swa<acronym>, the same way as it's done for courses and communities.
And then, through the package parameters, set a different value for those parameters to see admin pages in a different color than courses and communities ones.
I can commit this to cvs head if there is anyone interested on it. Unless it was better to have it on oacs-5-4 branch.