So you want to change background color for the nav bar but not the image? In that case you are probably best using the per-category stylesheets.

Smart BG requires an image filename to exist for each page/category/etc. that it sets a class for. The intent of the mod is to switch images for desired pages, and this is method to determine which pages get different images, and a failsafe to avoid switching to a nonexistent image and getting a blank background.

The mod does not require that the smartbg_xxx.gif be used anywhere, only that a file with that name exists; so you could copy & rename one little file to all the desired category ids, and use a different constant bg image and changing bg color.