My recommendation is to choose your PowerPoint theme prior to building your presentation, because much of the placement of presentation elements will depend on the font sizes, placeholder locations, and background art of a theme. Changing from one theme to another after the presentation is built can, in some cases, be very annoying because it requires lots of fine tuned changed to the presentation that are time consuming. On the other hand, sometimes the change can be a breeze, and a new theme can breath new life into an old presentation. It is easy enough to preview changes, to see if you want to adopt a new one or not, and it is also easy enough to apply a theme to a single slide or group of slides within a presentation.
To change themes, open your presentation, click on the Design tab, and hover over a new theme in the theme gallery. The active slide in normal view will show a live preview of the new theme. If that looks OK to you, click the new theme and it will be applied to every slide in your presentation. Now review your slides. The new theme may or may not be a success. If you find a problematic slide, you can reformat it manually, or retreat to your former theme by simply clicking it in the theme gallery. (This goes for custom built themes as well, which will be listed in the theme gallery as long as you have built them through this version of PowerPoint. If not, navigate to them to reapply them).
As I say, the active slide will show a preview of the new theme. If you are just shopping the themes, select a slide that has a lot of components from border to border, top and bottom, and preview the new theme using this slide. This will let you know if any background art elements especially may overlay or obscure elements on your slide. If so, and you don’t feel like reformatting, then don’t use that theme.
If you want to apply a theme to a single slide, or a group of slides, select it (them) in the thumbnail pane and click the new theme, or right-click the theme and choose “Apply to Selected Slides.” If you use more than one theme in a presentation, and consequently more than one set of slide masters, when you change themes again you will have to do it for each group of slides from the original, and a single click will no longer set the theme for the entire presentation.
That’s it. Themes are an easy way to make dramatic changes to your presentation globally. In later posts I will demonstrate how to create and save custom themes. In the mean time, here is a video summarizing this post.