css modification site wide?

I have a multi-site installation .To take the advantage of functions.php .I create a plugin by my self using all the modifications in functions.php file .I activate it network wide and works well.

So when coming to .css file.I has to modify it per each theme .Could you tell me if I create a single css file and make it activated for site wide and also ability to switch to theme’s css file if necessary ?

Thanks in advance !