Minor refinements
Remove the key instead of setting it

When resetting colors back to defaults, we unset the configured value.
The defaults are already registered with NSUserDefaults at launch, so
this is a better solution. By removing it and using the registered
default, we allow the default to be changed in future even after the
user reset it.
Revert many of the previous commits

Instead of assigning one static default badge background color, we use
a clear color to indicate that we want to use the predetermined color
based on dark or light mode. Setting a global default proved
problematic as the same color did not blend well with dark and light
mode.
Fix Xcode warnings
Add small label to preferences
Minor frame correction
Fix text clipping
Define specific keys that can be removed through iCloud