As you can see I have split this from the original discussion as most of it was from last year and things have moved on.
I have tested this theme and do not see any problem in either the button of the icons.
I am using the most latest version of Theme Essential.
You can download the latest version here...
https://github.com/moodleman/moodle-theme_essential/archive/master.zip
EDIT:
The V2 Edit icons are an option.
Why Julian added this function is beyond me...all it has done is cause grief if this post and all the others associated with it is anything to go by.
My advice is stick to the old stuff that works.