Margins can be adjusted in your theme settings under Print Layout. There you can adjust the margins so that they're equal for the inside and outside for standard viewing on screen. You will need to readjust this before distributing for paperback or hardcover printing though.
You can make changes to your theme on the Formatting Tab by clicking the three dots next to the theme name and selecting Edit or Edit as New Theme.
For more details on how to set up or adjust your theme settings, you can refer to this tutorial: https://www.atticus.io/create-a-custom-theme/ It provides detailed instructions and tips on creating and modifying your theme.