Custom CSS Changes

Good Morning,

I am working on making some CSS changes for the images of the website. One change (attached) is the .widget title background.

I know how to use the theme editor and manually change the CSS file to make this change, but if I do so, and you update the theme, my changes will not be applied on the update.

Where can I go to make my custom CSS changes, but when the theme updates, my changes are not overwritten but an update?

Some theme editors allow a custom CSS area, these CSS changes take priority over the themes CSS and they are saved when updating the theme. How can I do this and not loose my CSS changes when the theme updates?