Imagine this scenario: you place a text box on your slide — then you change the color of the font, reduce or increase the font size, set autofit options, tweak the margins, etc. At this point of time you are happy with your settings. Then you add another text box in the next slide — and you realize that this new text box needs to look the same as the text box you created earlier! No — you do not have to change all settings again — you change the defaults within the active presentation so that any new text box you insert has the format abilities you want.
Learn how to set a formatted text box as your default text box in PowerPoint 2010 for Windows.

