If you are going to write custom code for the theme, please use Child Theme.
A child theme is a theme that inherits the functionality and styling of the parent theme. Child themes are the recommended way of modifying an existing theme.
Please download the whole package of PatioTime theme (not just the installable WordPress theme zip file), unzip it and you will find WordPress Child Theme > patiotime-child.zip. Install the parent theme first, then install the child theme, and activate the child theme.
Then you can change code or write your own custom styles for the theme in the child theme files. Later when you need to update the theme, just update patiotime.zip (the parent theme), so your custom code changes will not be overwritten (because they are all in the child theme).
You may like to read more regarding the child themes on official WordPress codex page: https://codex.wordpress.org/Child_Themes.
Please note: Writing custom code for your child theme is related to Item Customization which is not included in the support we could provide. More details can be found here.