I use Compare plugin 220.127.116.11 and Notepad++ 5.8.5 UNICODE.
Highliting of changes in compare plugin works only with Normal text syntax, but when we choose another file syntax (e.g. C# - no matter before or after comparing) background color of the whole changed line become as it specified in syntax style.
I tried to use ComparePlugin 1.5.4 with no success.
I'm sure it worked with previous versions of Notepad++ and ComparePlugin.
Highlight is broken since N++ v5.8.
I'm currently looking to fix it.
See my other reply here: https://sourceforge.net/projects/npp-plugins/forums/forum/730527/topic/4479662
you only need to manually update the plugin to version 18.104.22.168 .
Then it works!