Hi Ken
Hmmm... You could likely achieve it if you had two different skins and two different CSS files. You would generate WebHelp twice. One would use the Day skin and CSS and perhaps end up in a WebHelpDay folder. Then generate a second time with a Night skin and CSS and perhaps end up in a WebHelpNight folder. Then you would publish both sets of WebHelp output files to your web server.
Once that was complete, you would then note the path of each WebHelp start page. Then you could add a button to the WebHelp toolbar of each skin that just opens the other version.
If the WebHelp is linked to an application, I would think perhaps a developer could have some code to determine time of day and open the appropriate WebHelp.
Just some thoughts off the top of my head... Rick 