For any and all customisations that go beyond the Customizer, please consider a child theme for your changes. Using a child theme is quite easy and saves all your individual changes from being overwritten by a future update of your theme.
To help you get started a <themename>-child folder is included in your download package. In this folder you will find a ready-to-go functions file and stylesheet.
More useful information on child-themes can be found in the WordPress codex.