How can I change the fill of an entire series?

To change the fill for an entire series (or any range) of segments, you can select the first segment you want to change, and then Shift-select the last segment of the desired range. The selection of elements in a think-cell chart is similar to the selection of a range of files in Windows Explorer (see 4.4 Styling the chart).

If you have a legend displayed for the chart in question, you could also simply change the fill of the legend marker. The fills of the related segments will follow automatically.