The primary change between Mood-Log 2.1 and 2.2 is support for iOS’s Dark Mode. When your phone or iPad is in Dark Mode, the screens will be dark with light text.
The entry form in Dark Mode
The entry form in Light Mode
As a side effect of this support, some bugs were fixed where the text might have been invisible while you were in Dark Mode. Since the iPhone and iPad can switch between Light Mode and Dark Mode automatically (as a setting in Settings->Display and Brightness) this should fix instances where you could not read all the text.
There are some other minor bug fixes, including a drawing bug in the date range selector in the Summary and Export views, and updating the charts when switching between Pie Charts and Donut Charts.