I’ve removed the url() from “Header Styles” in customize.json and it has fixed it (you need to make a change in the Dashboard for it to take effect). Would like confirmation that I’ve not broken something else by doing this!
Before
"Header Styles": [
/* Background */
{ "selector": "#page #masthead", "type": "background-color", "label": "Header", "template": "$selector {background: $value url(); }" },
After
"Header Styles": [
/* Background */
{ "selector": "#page #masthead", "type": "background-color", "label": "Header", "template": "$selector {background: $value ; }" },
