Customization Questions- Alternate CSS file possible?

I see in the list of features of both the free and pro versions it says
Customizable template files

What does that mean exactly?

Is there a way to make a css file for the calendar grid that won’t be overwritten with future updates?

Our theme allows for editing CSS centrally but I can’t seem to make changes stick. Please don’t suggest the WPFull Calendar plugin to liven up the style… it doesn’t work correctly on the site – perhaps a conflict with the theme… I don’t know what the issue is but it seems like overkill if I only want to add a few changes to lines and header cells… nothing drastic since our focus is more on the content of the cells. Also want to CENTER the content in the cells without a lot of padding and I’ve tried everything. Will keep at it.

Almost ready to go pro… if we can get these few things ironed out.

http://wordpress.org/plugins/events-manager/