The best way to CSS the WP widget to keep style under any theme

[an212CEARGYHYDAY]

I'm going to make WP widget plugin and want to make it theme independent.What I mean about this is the fact that I need this widget to keep style set in its CSS regardless of theme used. I'm worrying that theme will overwrite its style.What's the best CSS technique to do this ? Should I use style based on widget ID(#) or its Class(.) ?