Hello There,
Thanks for writing in!
In the customizer, there is two custom css section. The Additional CSS which is a default from Wordpress and the theme’s custom css section. The theme options custom css can be exported along with all the theme settings by using the Customizer Manager in X > Overview > Customizer Manager. For more info, please check this: https://theme.co/apex/forum/t/addons-customizer-manager/77/1
If you have place a custom css in Additional CSS section, you will need to manually copy your custom css and paste it a new file and save it as your backup. You can also do this if do not want to use the customizer manager.
Hope this helps.