How to change base url?

I’m creating a theme and used this guide to set it up locally:

I’m using VS code server to edit my files.

Now my base url for my ghost editing folder is something like this:
domain.com/proxy/port/

Problem:
Everything leads back to domain.com/
For example the css, js, images, etc will lead to domain.com/assets/...
Instead of domain.com/proxy/port/assets/...

Question:
How can I change the base URL to domain.com/proxy/port?
I tried to change the routes.yaml, but that didn’t do anything for me, perhaps I’m using it wrong.

You have to use relative paths, never with the domain name.

For your assets example, just use helpers:

Thank you for your reply.

I understand how the relative paths of handlebars work, but unfortunately with vs code server they don’t.
Meanwhile I sort of “found” the solution, but unfortunately I don’t know how to apply it to ghost or handlebars.

image
(this is fresh install)