Ok, I see that for the search results I have hardcoded the colors in the plugin's stylesheet. The reason is that I am using different color to indicate the line that is currently selected with up/down arrows and another color to highlight the line under the mouse pointer.
Now, while I have access to CSS variables defining colors used by Joplin, I am not sure how I can derive these 2 other colors at runtime.
I'll see what I can do here.