Custom theming of Nextcloud

I followed the installation instructions here for docker. I am using Docker-compose and my system is on Windows 10 Pro (1803) using WSL (Ubuntu 16.04). I am wanting to customize (more than just basics) my Nextcloud instance. How can I access the docker container’s data in my windows file system? My docker-compose file looks like this.

version: '3.2'

services:
db:
image: postgres
restart: always
volumes:
- db:/var/lib/postgresql/data
environment:
- POSTGRES_DB=nextcloud
- POSTGRES_USER=nextcloud
- POSTGRES_PASSWORD=nextcloud

app:
image: nextcloud
restart: always
ports:
- 8080:80
volumes:
- nextcloud:/var/www/html
environment:
- POSTGRES_HOST=db
- POSTGRES_DB=nextcloud
- POSTGRES_USER=nextcloud
- POSTGRES_PASSWORD=nextcloud
depends_on:
- db

cron:
image: nextcloud
restart: always
volumes:
- nextcloud:/var/www/html
entrypoint: /cron.sh
depends_on:
- db

volumes:
db:
nextcloud: