Issue with directories and files - customize themes

I would like to be able to customize the look and feel in grafana (the dark and light themes), but reading on the internet about this topic I see that the following files need to be modified:
1 - grafana/packages/grafana-ui/src/themes/_variables.dark.scss.tmpl
2- grafana/packages/grafana-ui/src/themes/_variables.light.scss.tmpl
3 - grafana/packages/grafana-ui/src/themes/dark.ts
4 - grafana/packages/grafana-ui/src/themes/light.ts

The problem is that these directories and files do not exist in my installation of grafana, I only have the sass files: grafana/public/sass/_variables.dark.generated.scss and _variables.light.generated.scss (and these field must not be modified). Too when I try to rebuild the front-end with the next command “yarn start” the following error is displayed: package.json not found.

Please, I would like to know why I am missing so many directories and files since without them I cannot customize aspects like the look and feel of the application. The version I have installed is the open source version 7.1.1