Logo size blurry

I try to insert logo in the header with standard resolution, but I think the theme i use (Astrid) forces a resize to a smaller version. I would like my standard resolution. I try to put in CSS code to adjust size, but it gets blurry when I make logo bigger. I think this is because of the theme, which CSS is prioritised before my own separate CSS. Anyone have experience with this?
Sorry for the bad english by the way…