Take the easy way out - announce that underscores can't be used as 
part of a theme name.

See: http://wiki.horde.org/Doc/Dev/Themes

When installing and using a theme that has an underscore in its folder 
name (I tried wps_sober which was targeted at Horde 3, downloaded from 
http://www.w8.se/tag/horde/ ) then the processing of that underscore 
causes trouble on some pages. A typical example comes from the column 
headers of the nag task list (but the problem is generic in nature, I 
believe, and not linked to the nag application). This is a section of 
the generated HTML code using the standard theme:

<a href="/horde/nag/list.php?sortby=completed&amp;sortdir=0" 
class="sortlink"><img alt="Afgerond?" 
src="/horde/nag/themes/default/graphics/checkbox.png"/></a>

This works fine, showing the checkbox.png icon and it allows to sort on it.

That same line, when using the wps_sober theme, becomes:

<a href="/horde/nag/list.php?sortby=completed&amp;sortdir=0" 
class="sortlink" accesskey="s"><img alt="Afgerond?" 
src="/horde/nag/themes/wps<span 
class="accessKey">s</span>ober/graphics/checkbox.png"/></a>

As you can see, there has been a parsing step that trapped the 
underscore and converted it into an access key "s". It is quite 
obvious the above does not work anymore. No icon is shown, only a 
concatenation of text parts from the above line.

I see 2 main kinds of solutions:
- adapt the parser code that generated the accesskey sections in order 
to avoid that it parses quoted src= or similar constructs
- set forward a standard that forbids the use of the underscore in the 
folder name of a theme.