I can confirm the attachment problem with firefox 3.6.8 and I.E. 8. I have worked on the problem a little, and I see a difference in the /templates/compose.html file, where the attachment window is set to popup class. The default theme does not have that. I'll try to look into this problem and post what I find here.
I also noticed that (and this is more of a roundcube problem) the user preferences --> interface skins does not include all of the themes. If I set one of these themes to default in main.inc.php, I can see that theme and the default theme in the interface skins drop-down list, but if I select default and click save, the custom theme disappears.
The theme pack looks awesome though! Thank You!