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 template, and Premium users and above can change the color palette in the Style tab. If you don't want to change the color entirely, you can also create a new scene and look for a scene design with your colors applied differently.
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.