Summary: * Replace remaining checkerboard images with repeating-conic-gradient CSS. Note that this is a trade-off between improved Dark Mode and browser compatibility: Such CSS is only supported since Chrome 69/Opera 56 (09/2018), Firefox 83 (11/2020), Safari 12.1 (03/2019), and Edge 79 (01/2020) according to https://caniuse.com/?search=repeating-conic-gradient. Older browsers should see the PNG image file instead, with no Dark Mode improvement here. In the special case of a checkered tag this may lead to decreased readability on such older browsers. * Introduce Checkerboard patterns variables in CelerityPostprocessors to use the very same two colors for the three checkerboard patterns as the PNG files do. * Introduce separate text and icon colors `sh-checkeredtext`, `sh-checkeredicon` for checkered tags (was using grey before) to keep text readable on a dark checkerboard pattern background. * Do not hardcode text-shadow color for checkered tags but use a variable. * Replace hardcoded white border color with a variable. * Ignore `alphablack` shadow for now - looks okay. See similar rP03e8c8ab0cdb. Refs T15056 Test Plan: * Go to http://phorge.localhost/settings/panel/display/ and set `Accessibility` to `Dark Mode (Experimental)` * Go to PNG image with transparency http://phorge.localhost/F1 and hover over it * Embed a PNG image file with transparency via `{F1}` markup and hover over it. * Go to the "Shades" section of http://phorge.localhost/uiexample/view/PHUITagExample/ and look at Checkered tag Reviewers: O1 Blessed Committers, mainframe98 Reviewed By: O1 Blessed Committers, mainframe98 Subscribers: mainframe98, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno Maniphest Tasks: T15056 Differential Revision: https://we.phorge.it/D26426