Fix: Background not showing for HACS themes.

Step 1
Create the following folder:
mkdir /config/www/
Step 2
Create a folder for your theme:
mkdir /config/www/theme/
Step 3
Copy your images into this folder.
The /config/www/ folder will now be refered by Home Assistant as:
/local/
Thus, the theme folder inside it will be referred by Home Assistant as:
/local/theme/
Step 4
Edit your theme's .yaml file from:
background-image: "center / cover no-repeat fixed url('/config/themes/theme/night.jpg')"
to:
background-image: "center / cover no-repeat fixed url('/local/theme/night.jpg')"
Step 5
Restart Home Assistant.
