It's a verb, "to rice" means to deeply customize your system to suite your needs and/or preferences. Install a desktop environment, setup it however you like it, add panels with system information, etc etc.

The more h4xx0r-like and minimalistic, the more street cred you get.

See https://old.reddit.com/r/unixporn (or https://reddit.com/r/unixporn without an account).