I am very frustrated that Samsung took away the ability to change the clock style on the always on display. Since I have a theme applied, the AOD picture displays on the AOD and the clock is very small. Before a recent update, I was able to change the clock style on the lock screen which would also change it on the AOD screen. It would then no longer display the picture, and the clock would be bigger. With my older eyes, I'm barely able to see the clock numbers.
You can change the clock size, font snd color on the lock screen. Long press on the lock screen to edit. Long press the clock and the clock style screen will come up. You can grab the edges of the clock box to enlarge, you can pick the number style to something easier for you to read, you can change the clock style from the selection and, at the bottom is the color selection. When you are done, click done at the top to save your changes.
