[dev] Silver Theme

[Chuck Hagenbuch]

Quoting Jan Schneider <jan at horde.org>:

> This is a broader problem, because specifying inside a Horde file
> which modules provide a certain theme is a BC break in any case. I
> can't remember whether we discussed this when introducing
> $theme_icons. It's safer to check whether a theme directory exists
> inside an application's directory.

This change solves the problem for me. If it looks good let me know  
and I'll merge it:

http://cvs.horde.org/diff.php?r1=1.305&r2=1.306&f=framework%2FHorde%2FHorde%2FRegistry.php

-chuck