First image is my Netbeans IDE. As you can see whenever I place the text cursor on a variable, that variable is highlighted everywhere else where it appears.
I find this feature very helpful and want to enable it on Android Studio (second image).
Have you tried
Identifier under caret setting?
Probably some mismatch with the colors you have set for Foreground and Background
Check my screenshot and let me know how it goes
Have you try this =>
File -> Settings -> Editor -> General -> Highlight on Caret Movement
Then for my personal use, I checked the three options :
-Highlight matched brace
-Highlight current scope
-Highlight usages os element at caret