Subtle visual tweaks to the way highlights are drawn in the Outliner in order to improve readability. * Similar to the File Browser, draw highlights as rounded corners. * Draw an additional slightly lighter outline in active elements. * Replace the dark background for dragged-onto elements with a lighter color, and add an outline. See PR for details and screenshots. Part of #135192 Pull Request: https://projects.blender.org/blender/blender/pulls/140371