The problem with DarkB theme and many others that John Stabinger makes is that the have wide side margins, so you lose nearly two inches off the screen to start with, which is OK with a wide screen, but frustrating with a small screen. I always use 1024px by 768px and I don't have any problems editing on my site, but then I don't use the DarkB theme LOL.
Have you thought of using another theme?
Getting back to your site, and the problem you face, could be in the settings of the theme. So have you tried doing the same editing in Standard theme?
A quick way to test this is to set Allow theme change by URL in the Theme settings page. Then you can select standard theme by just adding ?theme=standard after your main site URL...for example:
is my site and this link make forces it to fire-up using standard theme. It goes back to your normal theme when you log out.
Now if you tried editing in Standard theme, and all was well, then ther is a chance you can fix the theme with CSS, but not having darkb on my site I can't say what those CSS rules would need to be, but it should be possible.
I'm not at my desk right now I am testing on a Laptop running IE6 which is fun! So I wont be able to test my ideas about your theme till later today, unless you send me a link to your website and give me access to a test course of something? If this is possible send me details in a message.