I mean really - if you google free bb themes, tons come up - I know installing one that's not made for your model is bad but how do you know if themes are safe?

You don't.
If you don't trust the source, then don't install the code on your device.

I've NEVER heard of an instance where a Theme will "kill" your Berry...UNLESS the user installs the wrong theme for the Berry.

Take the 8700 for instance. Most 8700's are on the 4.1 OS. Themes built with Plazmic's ThemeBuilder ver 4.2.2 for the 8700 series can only be exported under OS 4.2.1. If you install a theme with a higher OS than your Berry has, youo may brick your Berry.

Other than doing something like that...I've never heard of anyone creating a theme only to harm a BlackBerry.