Base theme stylesheet overrides with a differing directory structure.

I came across an issue with stylesheet overrides of a base theme with a subtheme with a differing directory structure. If a subtheme's stylesheets reside in a different directory than that base theme's, they will not be overrided.

Example:

Zen has all stylesheets in the root directory of the theme. If I want to make a subtheme of it and override some of the stylesheets, I have to maintain that exact directory structure. If I place my stylesheets in a folder named "styles", those stylesheets will not be overridden since the keys in the array do not match.

I have created a patch for 6 and 7 here since I am not sure if it is a good idea to change it in the current version. I personally consider it a bug since it does not seem to match what happens with module stylesheet overrides, but it can break a lot of sites out there.

The fix to me seems quite simple in both versions.