Hi Dan,
I believe the Search dialog uses system level fonts so not configurable within MLO. It looks from your screenshot like you might be using Linux. Not sure what your options are within Linux for system fonts and I'm guessing you're using Wine to run MLO so it may be problematic anyway. One option in Windows is to use the built in screen magnifier accessibility tool - I'm sure there will be a Linux equivalent. For day to day use though this probably wouldn't be practical.
Personally, I maintain a dedicated search tab/view for the whole outline and use the Text filter section in the Filter Pane. That way I get results in the Outline (using the Outline font). The next desktop version of MLO is due for a UI overhaul so, hopefully, we'll see some improvement in this area.