You could paint the treads a warm gray or medium gray-green, so they won't be as vulnerable to looking dirty, if they're used all the time. Square balusters are fine - they were sometimes used in simple Colonial buildings, and can be relatively thin. You can also use simple turned balusters.

Are the spindles used here square? I don't want country looking spindles but am unsure if the square would look right with the curve of the staircase. Here is a picture of the actual staircase I'm working with. We're pulling up the carpet and the flooring needs to be refinished and will most likely be left natural red oak because we need to lighten up the space.

The balusters in our picture are square. When you redo the stair, you might consider removing the straight handrail on the wall, and extending the open-side railing with a "dogleg" piece. The railing will have to be removed in order to install new balusters anyway.