lib/theme: shfmt && shellcheck

My apologies to future `git blame` hunters ♥

lib/theme: don't redefine battery_char()

Combine the two definitions for `battery_char()` so the second one doesn't just overwrite the first one. Do one or the other, not both.

Don't evaluate if `battery_percentage()` is available at load time, evaluate it at run time.

Author: gaelicWizard