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Just after downloading the template, save it (in the save as box, choose PPT
template). It will be stored in the templates folder automatically. Then
open the already created presentation and go to the Templates task pane,
click on the downloaded template, or click on the listarrow next to it and
choose to apply on selected slides or all slides. If you do not see it in
the list of templates which are sorted according to color, click on the
browse link at the bottom of the task pane and choose it from there.