When in text editing mode, simply click-and-drag your mouse cursor to select important keywords, then click on the Highlight button to apply a highlight color to the selected words.
Want to highlight more than one chunk of words? You can do that! Jump back into text editing mode, highlight some more text, and click Highlight once again.
The color of the highlight is controlled by the theme, and Premium users and above can change the highlight color under the Style tab. You can also click on Layouts on the side of a scene to look at additional options for how color is applied to your scene.
To remove text highlight, you can click on the "clear highlight" button to remove it all, or select just the part you want to remove, and click on the highlight button once more.