Clarity - avoid being fancy by using a font style that is easy to read. Make sure that it is big enough to be read by the audience. A font size of 72 is about an inch (depends on the screen size). A one-inch letter is readable 10 feet away; a two-inch letter is readable 20 feet away
Simplicity - use bullets or short sentences. Summarize the information on the screen to have your audience focus on what the speaker is saying than on reading the slide. Follow 6-7rule (6 lines 7 words)
Contrast - use light font on dark background or vice versa. This is done so that it is easier to read. It is easier to read on screen if the background is dark. This is due to brightness of the screen