Adding custom js and css in a child template J5

[pl71]

So, what is the best approach to add some js and css files in a child template?
I'm trying with the 'joomla.asset.json' file.
But this file is located in the parent template folder (templates/cassiopeia), shall I have a local 'joomla.aset.json' file for my child template? When I created this child template, such a file didn't duplicate.
I want modifications to be specific for this child template only.
Is there a better approach? By better I mean fast in the first place, so I prefer writing some code, than installing extensions and plugins.

[pablop76]

add the user.css file to the css folder and/or the user.js file to the js folder in the child template
https://docs.joomla.org/J4.x:Cassiopeia ... tomisation

[pl71]

Yes, this is obvious. Presently I have user.css and I added custom CSS code to the bottom of the file.
This is not so hard and you will end with only one CSS file for faster startup.
The downside is difficult to update - you have to locate custom code in user.css and replace it with a newer version if any.
So probably I will keep 2 CSS files, but the question is how to make that nicely and efficiently.