Hi Knud!
There is no option for this one on the back-end, but with a simple custom CSS rule it can be changed easily. Try this one:
[html]div.ajaxsearchpro[id*=’ajaxsearchpro’] .mCSBap_scrollTools .mCSBap_dragger .mCSBap_dragger_bar {
background: #3a3a3a !important;
box-shadow: none !important;
width: 8px !important;
}[/html]
This will give a dark look for the dragger rail, like so: https://i.imgur.com/bypoOoA.png
You can apply this code to your theme custom CSS field (if it supports it) or use the custom CSS field on the search plugin back-end.
I hope this helps!