Load specific stylesheet

Hello, here Hanneke again,
I have a question about adding css to blocks i have created my own.
In the documentation I find I have to add a less. file to the folder templates/t3_bs3_blank/layouts (see attachment).
But this folder does not exist.
How can I add css to a newly created block?

I'm working with version 2.2.1 of the blank template.

Best greetings,
Hanneke