This custom theme is for "Checker Plus for Google Calendar™"
(hereafter referred to as "the extension").
which is a extension of Chromium based browsers
i.e. Google Chrome or Microsoft Edge
You can get the extension via
jasonsavard.com
After installing the extension
Just copy&paste content of Gray.Css into the CSS textbox and apply it
and you will see this theme applied to the extension
Unfortunately,
due to some native bugs and Shadow DOM issues with the extension,
difficult to be overwrited by limited coverage.
This theme is not perfect match to all elements;
Please modify the CSS yourself accordingly,
or report your problem to the author of the extension
if you encounter any problem.