When I first download macOS 10.12 Sierra I noticed that Quick Calendar looked a little out of place. While all of Apple’s Notification Centre widgets had a light theme, Quick Calendar had a dark title bar and looked much more like things did in Yosemite and El Capitan.
I re-build the widget for Sierra. No change. I looked in the documentation for style and theme changes. Nothing. I thought, perhaps, that Apple had kept the light theme for themselves. Luckily I was wrong.
And this new version of Quick Calendar has that tiny update so that it looks at home on macOS Sierra. It still works on El Capitan and Yosemite.
I just submitted it to Apple. It should be available to download in a few
days hours. Enjoy.
Update: It was approved last night, less the six hours after I submitted it!