Four of them
A theme is not a colour swap. Each one changes the palette, the lock-screen scene, the card style, the message sounds, the vibration patterns, the charging indicator and the chime when you plug it in. Every screenshot here came straight off the device.
Squatch
terminal greenPines, mountains and a sasquatch walking the ridge. Clean sine chimes, a steady double buzz, and a lightning bolt on the battery while it charges.
Blocks
grass, dirt, pixel skyA blocky world under a pixel moon. Soft decaying piano notes, quick taps like chipping at a block, and a stepped pixel bolt when charging.
Hero
night hills, heartsA castle on the horizon, a hooded figure, and a row of hearts that tracks your battery. Square-wave fanfares, and the charging mark is a little heart.
Aurora
northern lightsCurtains of light over a dark ridge. Slow bell tones, long gentle vibration, and a four-point star on the battery that drifts through the sky's colours while it charges.
Switching
On the pager: Settings → Theme. It takes effect immediately, including the sounds and vibration. The lock screen also carries a rotating one-liner, and each theme has its own lines mixed in with the general ones.