i mean the popup windows that appear when you click on some items in the wysiwyg editor..for example, the External Link helper.
I'm using Firefox on OS 10.6. The behavior (white on white) is also present in Safari.
Darkroom gave me trouble in Tiki4. The tabs were not behaving correctly.
Tab behavior shouldn't be theme-specific. The theme stylesheets only have rules for visual appearance, not functionality. This, and the fact that we can't replicate your problem (I also asked another person who can check with MacOS and its browsers) suggests to me that there might be an installation or configuration problem at your site.
Could you update your site to Tiki 4.2 - a good idea anyway to get security and bug fixes - and see if the problem persists?