Themes include settings for colors and fonts. A theme’s color scheme affects most elements on your slide, including background, text, and graphics. Themes provide sets of colors that work well together so your slides should maintain an attractive and cohesive look no matter which one you apply. Font schemes include settings for heading and body fonts. When you apply a theme, it contains a preset color scheme and fonts, but you can change these to give your slides a new look.
Be sure to use color combinations that work well in the space where you will give your presentation. Most color combinations PowerPoint offers should help to keep your text readable, but if you decide to change the color of an individual element yourself, remember that dark text colors are hard to read in a lighter space.
You can also apply theme elements to all slides by applying them through the Slide Master. For more information about working with masters, see "Display and Navigate Masters".