How to add to or override part of a class definition

In my file, I have a pre-set definition of a class, say ".nav", and it has a number of properties:

.nav {position: relative; width: 980px; display: inline-block; border-bottom: 1px solid #126a13; padding: 12px 0 8px 0; background: url('images/nav_bg.gif') repeat-x top center; clear: both; text-transform: uppercase; line-height: 1em; font-weight: 400;}

Lower in the page, I want to be able to replace just one property, the background, with a new value e.g.

.nav {background: url('../../../image-new/nav_bg_new.gif')}

The problem is that it seems to replace the entire first definition of .nav, instead of replacing just the one value, for backgroun. Alternatively I can get the desired effect by repeating the entire first class definition with the one value changed, but I wonder if there is a way in CSS to say "keep all the existing properties, just replace this one". Thanks in advance.
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DavidInsight and Energy SpecialistAsked:
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Dave BaldwinFixer of ProblemsCommented:
"keep all the existing properties, just replace this one".
No but that is the way it works.  However the original background defintion is "background: url('images/nav_bg.gif') repeat-x top center;" and your replacement is "background: url('../../../image-new/nav_bg_new.gif')".  Note that you do not have a ';' at the end of your replacement.  Under some circumstances that can throw things off.  It never hurts to add it like "background: url('../../../image-new/nav_bg_new.gif');".
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DavidInsight and Energy SpecialistAuthor Commented:
Excellent, and thank you. I had not noticed the additional values that I omitted. I also added the trailing ";" to be safe. Now everything works and is cleanly coded. Much better. If interested, the site is http://tildet.com.
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Dave BaldwinFixer of ProblemsCommented:
You're welcome, glad to help.
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