Recently, I installed the Neon.3 updates. It seemed to crater the Darkest theme. I was originally planning to come here to ask if others had seen this and how to fix it. However, I just managed to get back in working order by taking the following steps:
Switched back to one of the standard themes
I was getting errors from the code recommenders plugins as well. Maybe unrelated, but I uninstalled them.
Edited eclipse.ini to add the “-clean” parameter
Switched to Darkest theme and it worked!
Edited eclipse.ini to remove the “-clean” parameter and restarted Eclipse again
If others are having issues it may be worth trying these steps or some variation involving the “clean”.