Hi Michael,
As James has said this should be fixed in v6.5.7. FireOS devices (Amazon tablets) were having issues with the on screen keyboard not showing up when using text boxes. I put the long press action as a temporary workaround in v6.5.6 to force this, but in doing so broke the normal long press to select method. I reverted the behaviour in v6.5.7
Regarding the theme change, this was unrelated. The two biggest criticisms I get in the app reviews on the Play Store is the complexity of the app (not simple to use) and that it looks like an old MS Dos program - mostly due to the dark text based menus, etc.
Personally, I like dark themed apps and thats how the app menus always appeared (regardless of the song display theme dark/light/custom1/custom2). The app always defaulted to the dark theme when booting up. If a user had manually changed chosen a theme (i.e. not relying on my default), the theme updates I made would follow this theme. If you had only ever used my default start up theme (i.e. never tried to change it), the recent update would default to your device settings on first boot (viewing apps in light or dark mode - sometimes called day/night). Most users leave their devices in day/light mode, so I decided to make the app default follow the device settings on first boot. The app then stores this as your default. I think this is what has happened in your case - your device is set to day/light mode and you have always just left the app running in my default dark mode - the update assumed you likely prefer a light theme.
Best wishes,
Gareth