Hi everyone. I'm wondering if there is a way to select many shapes at once (which are all different colors), and lighten or darken them all at once, WITHOUT having them all get set to the same color? The only way I know to change the shade of shapes is to adjust the sliders in the Stroke & Fill dialogue -- but that also resets all their colors to the same value...

I guess you already know how to select more than one object (using Shift for example), you can change the color this way using the Color extensions, under Extensions > Color menu. You might need to do it more than once, and select the objects after each change.