You can adjust the settings of your story by following the below mentioned steps:

Step 1: Open your story.

Step 2: Tap on the settings icon from the navigation bar at the top right corner of the screen.

Step 3: A menu will pop up. You can adjust your settings in the following ways:

It allows you to increase/decrease the font size of your story.

It allows you to change the white background to black and vice versa. 

It allows you to lock/unlock auto rotation.