Is it possible to change highlighting color?

Yes, you can change the highlighting color without transparency in CSS. Please find the following classes in flexmonster.css file and change background-color property the way you need:
     #fm-grid-view .fm-sheet-selection-canvas,
     #fm-drillthrough-view .fm-sheet-selection-canvas {
          position: absolute;
          background-color: rgba(255, 245, 106, 0.2);
          pointer-events: none;
     }