In PowerPoint Web App you can use the Font features on the Home tab to change the font color and size, and apply bolding, italics, or underlines to your text. To use WordArt text effects, you’ll need to open your presentation in desktop PowerPoint (if you have it), and add them there:
In PowerPoint Web App, click Open in PowerPoint.
On the Insert tab, click WordArt.
For further help using WordArt in desktop PowerPoint, see Insert WordArt.