Combine drawable files. https://redmine.stoutner.com/issues/794
Update the URL in the copyright header. https://redmine.stoutner.com/issues/796
Bump the minimum API to 23. https://redmine.stoutner.com/issues/793
Switch to the new Day/Night theme. https://redmine.stoutner.com/issues/522
Add setting to disable screenshots. https://redmine.stoutner.com/issues/266
Create a dark theme for `SettingsActivity`.
Create a dark theme for `GuideActivity`.
Add controls for `Clear and Exit`. Implements https://redmine.stoutner.com/issues/133.