I have seen reports of the Clock panel background color not working in previous releases. I assumed it would be fixed with v7 but it seems not. Any chance this can be fixed in a future release?
I found out this issue was fixed in clock-panel 1.1.1 but it did not update to that version automatically with an update to Grafana v7 due to another bug. But the devs have now fixed that issue so I can confirm as of 22 May 2020 grafana-cli plugins update-all will update the clock panel correctly (to the current 1.1.1 version).