First, you may want to have a look at the Appearance settings. By going to the Keeble app, and then selecting Keyboard Settings > Theme > Custom Theme, you can choose your own colors for all the different types of keys (e.g. high-contrast colors that suit the user best) if any of the 5 included themes don't meet the user's needs
Now tap Keyboard Settings, scroll down to the Access section, and make sure that Alternative Access is ON. Next, tap Select, and then tap Select on Release. When Select on Release is ON, Keeble keeps following your finger as it travels over the screen, and only decides which button should be selected when you lift your finger off the screen.
While traveling over the screen, it will mark which button you are touching with your selected Tracking Color and Cue Width, and can even speak each letter that you are touching when Spoken Cue is ON. After making a selection, the key will be marked with the Confirmation Color.
You may also want to check out the Auditory Feedback section. Speak As You Type can give the user confirmation that he or she is typing the correct letters and words. Keeble can speak each key, word, or sentence. In Voices and Speech Rate, you can personalize how Keeble speaks as the user is typing. Note that Keeble's voice feedback does not work when your device is muted.
If the user benefits from Word Prediction, change the Prediction Height to Large and change the Max Numbers of Suggestions to the least suggestions possible. Both features will make it easier for the user to see and access word prediction.