Recently I got annoyed using the find feature in the Preview.app and not being able to see the found strings easily.

I found the answer here. There are two parts to it

1. first one can change the highlight color used to display the found text in Preview.app with the defaults command like so
   
   defaults write com.apple.Preview AppleHighlightColor "0 1.0 1.0"
   
   which will result in an ugly cyan. Presumably the colors are to be specified with values for red green and blue ranging from 0 to 1.
2. one has to TAB into the window pane containing the searched text because if that window is not active the highlighted text will be displayed with the inactive window highlight color

What a pain.