Two things:

1) THe radio station I want to listen to is 88.7, but it will not go down farther than 88.9... How can I tune in to this station?

2) is there a way to activate night-time colors without having the headlight switch / dimmer hooked up? Is there a button to do this? There should be...

#2, night color scheme is in settings, navi or map settings.

There is automatic (chosen by time and illumination wire), day, or night.

You can manually set the dimming on whichever mode you are in.

It's all in the manual.

1) THe radio station I want to listen to is 88.7, but it will not go down farther than 88.9... How can I tune in to this station?

That's odd. I can tune to 87.9 on mine.

Assuming you are in the US - or have a US Z1x0 - Pioneer says the range is 87.9 - 107.9

Try going into the FM settings (bottom of the screen) and try different sensitivity settings. Also try toggling "Local".