How to style notifications on websites?

Sometimes I need to display notifications to the visitor, but the styling I’m currently using is just plain ugly (I’ve taken a screenshot of a particularly destructive example).

Not being a designer, my question is this: how do I make this good-looking?

Note that this one is green (for “success” message), there are also “error”, “warning” and “tip” messages which will probably need other colours, though if people can think of other distinctions that’s cool too.

TIA.